Stealth Sonics C9 – where rocket science meets rock and roll

Pros: excellent balance of detail and power, fuller and thicker bass presence than U9 with deep sub-bass, extended treble, Klarity Valve eliminates listening fatigue, good set of accessories, tough build and impeccably crafted

Cons: large price difference between universal ($999) and custom, trades some resolution for musicality, carry case doesn’t scream high end

Price: from $1499 upwards, depending on custom build options

Website: https://stealthsonics.com/

Rating

Introduction

I first came across Stealth Sonics back in Canjam 2018, encountering their very enthusiastic rep at the time Dindae, who made sure I heard all the various models they had to offer before I left that weekend. Suffice to say, what I heard did impress me somewhat, as I came home with both their U4 and U9 models from the universal range, which have been in my possession (and listening rotation) for well over a year now. The pitch for the company was interesting, emphasising Stealth Sonics as a firm which had roots in audiology, but also in rocket science, with a founder who used to be a NASA engineer.

They are looking to enter both the audiophile and touring musician markets (through their partnership with Rat Sound), and have already landed bands like Pearl Jam as users of their custom range, so are starting to pick up some traction. I have always loved manufacturers who look to innovate, and the work around pressure control in-ear (for long term ear health) and modelling of airflow between the drivers and the output nozzle (the NASA / rocket science angle) really interested me. I have always found that IEMs that have some form of balancing tech seem to sit pretty well in my sweet spot in terms of preference, so when the opportunity to hear the current flagship model in their custom range came about, I jumped at it.

Acknowledgement

The C9 was received free of charge from Stealth Sonics head office in Singapore for the purposes of review, and for comparison against their universal U9 model (which I purchased direct from them at Canjam London 2018). As a custom IEM, this does not have to be returned to Stealth Sonics once the review is finished. There has been no financial or editorial input by Stealth Sonics in any part of this review, or any incentive to post positive feedback, so all views expressed here (however misguided!) are 100% my own.

I would like to thank Raj and the rest of the team at Stealth Sonics for the opportunity to hear the C9, and for the answers to the various technical queries I had throughout the review process.

Unboxing and accessories

The C9 comes in a simplified version of the Stealth Sonics current universal packaging, arriving in a good looking black carbon-fibre effect presentation box. The box is about the size of a decent hardback book, and feels sturdy. Apart from the carbon fibre, the only branding is a subtle Stealth Sonics logo embossed on the bottom right of the box front. Opening the magnetic fastener, the packaging opens out to reveal the IEMs (sat in a laser-cut foam inlay) and the large leatherette carrying case. Like the box, the carry case has a Stealth Sonics logo on the metal plate affixed to the front, and the name of the client lasered underneath. The name is subtle, but will most likely help touring musicians (one of Stealth Sonics’ key markets) distinguish which gear is theirs when the roadies are unpacking the crates, so it is probably more functional than cosmetic.

Apart from that, the inside of the box is as spartan as the outer, with a small packet on the inside front cover holding a Warranty/”Thankyou” card (a nice touch). Another nice touch was the inclusion of a handwritten note from the Stealth Sonics team in the box thanking me, which I have seen on a few other Instagram posts of CIEM unboxing of theirs. It was a simple gesture, but very effective, highlighting the care and attention that goes into each customer.

As a custom IEM, the customer isn’t usually expecting much in the way of accessories, but Stealth Sonics do pack in a fair amount here. The C9s come with two 3.5mm terminated cables (one an “aftermarket” quality silver plated copper braid, and the other a slight;y thinner braided cable with an inline microphone. They also pack in a flight adapter, a 3.5mm to 6mm jack plug and the ubiquitous CIEM cleaning brush into the package, along with a neat Stealth Sonics branded cleaning/polishing cloth. Apart from the obvious lack of IEM tips, this pretty much mirrors the loadout on the universal series models, and compares favourably against a lot of the other CIEM packages I’ve seen to date. Overall, there is a feeling of simple refinement about the design, and the addition of two cables is definitely a nice touch.

Build and fit

Stealth Sonics make a big point on their website about the design of their custom IEMs, highlighting their connections and background in the audiology industry. They have partnered with various audiologists across the globe to obtain deep second-bend impressions, with the aim to place the nozzle of the CIEM as close as possible to the eardrum, avoiding any canal effects or disruption to the sound. Unfortunately being a Singapore-based brand there weren’t any audiologists listed in my particular area of the world (the UK), so my customs were made from a set of electronic scans I already had on file from a previous CIEM. I therefore can’t confirm what the exact fit would be like from a Stealth Sonics approved set of ear impressions, but I can confirm I am more than happy with how my C9s turned out from a “standard” set of impressions.

The all-acrylic build is smooth and flawless, with a highly polished proprietary lacquer finish which is designed to protect the body of the IEM from damage. Now, I tend to baby my gear (especially the £1k+ pieces!) so I will probably never have to take advantage of this, but if you are interested, you can check out a video of someone throwing their C9s off a staircase down onto a concrete floor without any visible signs of damage. Again, I have no intention of doing this so please don’t try this at home if you are a Stealth Sonics owner, but I can say that they feel solid and robust in the hand, and definitely not as fragile as other customs I’ve handled. The shells also seem to be a little thicker internally than the other CIEMs I own, which may also contribute to the feeling of stability when you take them in and out.

The finishing is excellent, with no visible raised edges or seams between the IEM body and the faceplate and no air bubbles. When I first met the Stealth Sonics team at Canjam London 2018, they showed me some interesting promo material about how their CIEMs are manufactured and polished by machine at a facility in Germany. Again, feel free to check it out on their website – whatever it is their doing, it seems to be working, as these are a seriously well made CIEM.

They have the now-obligatory custom IEM designer available on their website with various options available. I opted for a blue on blue colour scheme, with blue glitter shells and a blue abalone effect faceplate. The final result is pretty stunning (if I do say so myself), with the glitter effect clearly visible on the inside of the shells, highlighting the thickness of the external acrylic “walls”. The faceplate again seems to be presented under a layer of lacquer, with the logo I selected (a simple “JP77” on each shell) seeming to “float” a little above the abalone plate but below the surface of the IEM shell itself. It is a nice visual effect, and catches the eye more often than you expect when you are looking at it. There are a few other touches on the shell, with a small white Stealth Sonics logo engraved on the topmost surface of both IEMs, and the model number printed on the top of the inner face. I also opted to have my name printed on the inner face, and this appears in a simple white typeface towards the bottom of the IEM, near the spout.

In terms of comfort, the C9 is excellent. It fits perfectly, and slips in and out easily. It isn’t quite as comfortable in the ear as the porcelain construction of the Wavaya model I recently received (the Quadra), and actually has a slightly shorted stem despite using the same set of scanned impressions, but other than that this is an excellent fit in my cavernous ears. The fit and finish even extends to the cable sockets, with the C9 fitting my Dunu Hulk cable and a few other after-market options I have laying around with no major issues inserting or removing, and a snug and secure fit once the cables are in place.

Overall, I have no complaints here – the C9 is a visually stunning piece, and has a fit and feel that screams quality and attention to detail. Very nicely done.

Technology

Stealth Sonics proudly use the tag line “It’s Rocket Science.” for their IEMs, packing five (count ’em!) different proprietary technologies inside the custom flagship. For simplicity, the descriptions from their website are listed below:

Iso-Stealth: “Through our advanced audiology research, testing and international network of over 700 audiology partners, we’re able to go beyond our competitors to engineer ultra-precise, second-bend ear canal impressions to position our IEM within just millimeters of the eardrum. Wearers can listen at safer SPLs for a cleaner mix, with less ear-cavity interference and a consistent sonic experience every time.”

SonicFlo Acoustics: “We’ve harnessed our technology breakthroughs in the aerospace industry to optimize aerodynamics, aeroacoustics and airflow performance. We’ve also taken advantage of our extensive research in fluid dynamics principles to engineer our in-ear monitors for optimum sound isolation in any environment.”

Stealth Damping: “Acoustic refinements include extra-large bores that preserve airflow and sonic fidelity and advanced venting features to manage resonance.”

Klarity Valve: “The Klarity Valve is designed to release pressure build up in the ears arising from long use of IEMs. Without such a 1 way valve, the listener becomes exposed to pressure build up in the ears causing fatigue and hearing stress.”

Ultra-hard Impact Shell: “Designed to withstand pressures arising for accidentally dropping the IEMs and other typical accidents, the shells are coated with a proprietary lacquer that makes the shells extra hard.”

The universal range also lays claim to being manufactured with a proprietary composite material (Stealth Kompozit), but the custom versions are manufactured from more traditional acrylic and lacquer coated instead.

The upshot of all these? The CIEM purports to manage the airflow through the bores, reducing turbulence that can affect the sound. It releases pressure to the listener through use of a one way valve (in a similar but not identical way to the current 64 Audio APEX technology and the “Guvnor” valve tech used in by Hearwave. It makes the IEMs ultra hard (as already mentioned in the build section), and it places the nozzle as close to the eardrum as the impressions will allow to reduce unwanted canal resonance.

It’s an impressive collection of claims, and while I am sure it is all playing a part, it is difficult to know what exact difference some of the tech is making to the end user listening experience. One thing I am happy to unequivocally vouch for is the Klarity Valve tech, however – these are comfortably up there (or possibly even ahead of) my old 64 Audio U8 in terms of lack of listening fatigue. I can wear the C9s for hours without any noticeable pressure build up or fatigue – combined with the excellent isolation and fit, and this is an IEM that can be listened to all day without feeling like your ears need a hot bath and a lie down afterwards. It’s always good to see IEM companies pushing the boundaries of what can be achieved with different areas of in-ear technology, and I think Stealth Sonics are firmly on the way here to harnessing some very useful innovations.

Initial impressions on sound

The C9 is the flagship from the current Stealth Sonics range (as of June 2019), and is marketed as a monitor “for people who want the most accurate response possible, with a warm full-bodied bass and silky smooth midrange and highs.” For once, the marketing text is pretty accurate, with the C9 exhibiting a nicely filled out low end and some smooth but definitely reference class midrange and treble. The C9 carries a fuller and thicker bass presence than its universal sibling, giving a slightly fuller feel to the low end and slightly more mid bass impact. This adds a pleasant tinge of warmth to the sound, without clouding the midrange. The sound is pretty balanced overall, with a fairly flat response through from bass to treble, keeping all three frequency ranges fairly even and stage-neutral in terms of relative position.

Starting down low, the C9 has plenty of sub-bass, providing a deeply extended thrum when needed and harnessing the dynamic driver to provide a nicely physical vibration in the inner ear. This carries a decent sense of weight (especially compared to the universal model), but it isn’t a hugely visceral or slamming sort of presentation. In the context of this sort of “reference” sound, this works well, allowing just a little extra in terms of texture and physicality in the lower end of the sound without disrupting the overall tone or cohesion.

Midbass is full and maybe just a shade elevated, balancing off nicely against the sub underneath. It isn’t enough to be considered as a basshead sort of quantity, but there is definitely enough volume to make the music sound rounded and full on tracks where there is plenty of bass present. It doesn’t add more bass than is present in the mix, fading out quietly where there isn’t anything but adding the right level of oomph where required.

The midrange is clear and unbothered by the bass below, transitioning smoothly without any bleed or overlap. The notes are somewhere between medium and thick in terms of overall “size” to my ears, but despite that, there is a level of clarity and inner resolution that marks the C9 out as a top tier performer. Instrumentation and vocals sound clean and defined, with plenty of black space between each note. Much like the U9, the custom variant is good at exposing the finer detailing in the midrange.

In terms of position, the instruments and voices feel quite close to the listener, sitting somewhere between stage neutral and slightly forward for me. It isn’t a JH Audio sort of “in your lap” intimacy, but the C9 definitely pulls the listener towards the music.

The C9 possesses a smoothly resolving sound in the midrange, picking out fine detail but not at the expense of shedding body or introducing harshness – it has the same feel of “inner resolution” that you get with something like the Empire Ears Zeus, achieving clarity through innate technicality rather than a push in certain frequency bands.

Treble is similarly proficient, again extending in a fairly linear fashion from the mids all the way through to the ultra highs. This isn’t an overly bright monitor, and follows a similar sort of path to a lot of “stage tuned” IEMs, with treble that is technically capable but not screamingly hot or emphasised. Extension is good to my ears, and the C9 maintains emphasis up past the limits of my hearing without any particular rolloff.

Overall, the C9 paints a musical picture that is balanced but still full, with a satisfying bass thump available from the dynamic driver when needed and some serious detail and resolution in the upper ranges. It doesn’t emphasise or de-emphasise any particular frequency bands, retaining a fairly flat tuning all the way through.

Bass

The dynamic driver powering the hybrid element of this IEM is a pretty capable 10mm unit, giving a good physical foundation to the presentation. It is very carefully integrated with the rest of the drivers, being tuned to offer more speed and a little less presence than you might expect, allowing the two driver types to blend seamlessly together in the soundscape without a telltale “crossover” between the DD-driven bass and the rest of the soundscape. The tendency on some standard dual-hybrids is to tune the DD for maximum impact without considering the knock-on effect that will have on how the listener will perceive the rest of the sounds produced by the less physical BA drivers. I’m glad the Stealth Sonics team went a different way here, as the C9 definitely benefits as a result.

“Heavy” from the last (and possibly final) Linkin Park album starts my bass testers, and the C9 handles it well, providing a solid but not overwhelming sub-bass rumble to the track. Ditto “Heaven” by Emile Sandé, the C9 producing a pleasant hum in-between the tears as the track kicks off. The sub-frequencies feel weighty but not over-emphasised, carrying the telltale physical presence of a dynamic driver, adding a sense of punch and visceral impact to kick drum strikes as the air slams into your inner ear.

As mentioned, this isn’t at basshead level, but definitely doesn’t feel lacking in decibels, pushing out the sub-driven lows in “Say Something” with a real physical substance. The weighting between sub and mid bass in terms of emphasis feels roughly equal, with only a slight mid bass lift compared to the lower range. “Get Lucky” by Daft Punk slinks through the bores of this IEM in classic fashion, the C9 playing the bassline with a liquid smoothness to the core, and allowing a fine coating of texture around the edge of each note.

There is a slightly warm softness to the presentation of some bass guitar licks that feels positively calorific in its richness. “Bad Rain” by Slash feels chunky and thick in the low end, trading in some of its trademark snarl and aggression as the bass kicks in for a meatier and slightly slower feeling tone. Drums still thud with impact on the track, so this isn’t an overly relaxed or laid back tone, but it definitely isn’t the snappiest response you will hear. This lends a more organic feel to the low end than the comparatively leaner U9 universal model, which is one of the main improvements for my tastes.

Detail in the midbass is good, the C9s dynamic driver doing well painting the finer textures on to the outer surfaces of the notes. “Hello, It’s Me” by Sister Hazel is a nice example, kicking off with a bassline so slinky and smooth it could run for political office, but still pushing plenty of detail and the fringes as the strings vibrate into silence. “The Chain” by Fleetwood Mac is full of definition, not quite giving the iconic bass riff at the 3 minute mark the weight it can carry with some single dynamic driver IEMs or planar cans, but still managing to raise the hairs on my arms.

“Reptile” by Skrillex is one of my testers for physical presence down low, and the C9 surpasses my expectations here, forgoing the softer presentation of the bass guitar for a more physical slam factor, pushing a good amount of air with each bass drum hit and snare impact. The pulsing bassline picks up some serious momentum as the track builds. This still won’t keep a true basshead from reaching for a little EQ down low, but for most listeners the C9 packs a more than adequate punch here.

Overall, the tuning here is pretty well done, presenting enough substance to give songs a real sense of “weight” when listening, but without overpowering the rest of the frequencies or unbalancing the IEM presentation. The mix between mid and sub bass also feels particularly well judged, the C9 kicking out a decent mid bass punch amid the detail and texture. This isn’t a strictly neutral tuning, carrying more bass emphasis than a truly flat monitor would, but the sound benefits immensely from that choice. I am a huge fan of natural sounding monitors, and the C9 comes up trumps here.

Mids

The midrange on the C9 transitions smoothly from the bass underneath, with no bleed or obvious crossover points. The bass pulls back just a shade in the transition, allowing the C9 to present the listener with a clean switch from the dynamic to balanced armature drivers. The C9 is one of the most coherent hybrid models I’ve heard, taking just enough body off the DD and matching the speed and tone of the two different driver types very well to present one unified sonic picture (a lot easier said than done).

Stage presence leans a little further forward than true stage-neutral, but still remains fairly close to the bass in overall shape, with just a shade of sculpting in the sound. This is a reference monitor at heart, and as such it remains close enough to balanced to keep those credentials firmly alive, without sounding sterile or unmusical.

Midrange clarity is good, with the C9 resolving multiple moving parts in busy tracks without any fuss. “Everybody Knows She’s Mine” by Blackberry Smoke kicks off with a good old boy Southern guitar riff and a honky tonk piano, layering some acoustic guitar on top at the 15 second mark. This can sound quite “blended” on less resolving IEMs, but the Stealth Sonics flagship pulls each strand convincingly apart, keeping the two guitar lines crisp and distinctly layered. It achieves this without unduly dissecting the music, giving each instrument a natural feeling of space without losing the body or fullness of the notes being played.

Guitar in general sounds good on the C9, the hybrid model providing plenty of chug to distorted guitar and a silky smoothness to more acoustic fare, capturing plenty of detail around the scuffing of strings and fretting of chords on tracks like “Nobody ‘Cept You” by Jack Savoretti or “Coco” by Foy Vance. The tuning gives both a slightly sweet sheen to the acoustic guitar, contrasting with the gruff and gravelly delivery of both singers, which retain plenty of the rawness from the original recordings.

Heavier guitar fare maintains a good sense of crispness and speed, carrying a decent amount of weight without blurring the sharp edges of each note. It sits pretty much in the middle of my personal sweet spot for rock style instruments here, giving an analogue sheen without sacrificing technicality. My favourite Slash test tracks are a guilty pleasure on the C9, “World On Fire” passing through in a blur of tightly controlled riffing, both crisp and liquid at the same time. Ditto “Shadow Life” very in the same album, the staccato rhythm that underpins the main body of the track stopping and starting with precision without sounding clinical.

Formative sounds on vocals are well defined, the C9 lending plenty of expression to Beth Hart’s smoky vocals, bringing plenty of emphasis and clarity to her breathy vocal style. It also keeps the sandpaper rasp of her higher end screaming on tracks like “Rhymes” with Joe Bonamassa under full control, with plenty of texture but no obvious harshness.

Looking for more signs of sibilance or rawness, “Whiskey And You” by Chris Stapleton is up next. The delicate guitar ballad sounds sparse and personal, dipping Stapleton’s vocals in a light layer of honey and avoiding any unpleasantness. This is both smooth AND detailed, and while the mastering isn’t the best example of studio craft you still ever hear, the C9 makes a good fist of making it listenable without overly smoothing the rough edges.

My other favourite track to test brittleness in the mid-to-upper range is “Starlight” by Slash. It kicks off with a deliberately dissonant and harmonic-heavy guitar intro, then explodes into life with Myles Kennedy’s high pitched falsetto. The intro is deftly handled, the notes feeling delicate and crystalline, with plenty of bite. Kennedy’s helium fuelled wail also comes through with plenty of edge, but has enough weight behind it to avoid grating like it can on some IEMs.

There is an almost analogue tonality to the vocals in this track, reminding me of the vocal tuning of the Flares Gold by Flare Audio. There is an inherent warmth and “roundness” to the sound that doesn’t clog up the space between notes, but provides a sense of body to accompany the obvious clarity and definition. This isn’t an IEM that will wow you on first listen with detail retrieval (its universal sibling the U9 is more likely to do that due to the thinner note presentation) – once you get used to the tuning, you will realise that the detail is all there, however, with the C9 rendering both macro and micro detail cleanly and precisely in the most involved of tracks.

Treble

This is the last part of the sonic puzzle, and given that six of the nine drivers used in the C9 are either dedicated to the high or “super-high” frequency bands, it shouldn’t be much of a leap to guess that they handle treble pretty well. The shape of the tuning feels pretty linear, extending up pretty much in a straight line all the way through my range of audible hearing and into dog bothering territory without any noticeable drop in emphasis. Due to this capability, there don’t seem to be a huge number of noticeable peaks in the response, the C9 remaining smooth and silky right up to the top rafters.

Playing “Chia Mai” by the classical fusion duo Duel, the opening violin lines sound strong and resonant, layering additional synth into the background as the track builds into a swirling mass of string instruments and delicate sparkle. There is bags and bags of fine detail, delivered with precision and clarity but no harshness whatsoever. In fact, for a CIEM with so much treble capacity, there is not a huge surplus of “bite” to proceedings up top. Stealth Sonics went a different way here, preferring a richer and more analogue tonality without the telltale bump in the lower treble that is usually used to give upper end emphasis.

This is an IEM for people who love treble, but not necessarily for trebleheads – capability is excellent, but quantity and emphasis are definitely more cultured than raucous or raw, caressing the eardrums with weighty treble notes and gossamer thin detail, but not carving into the musical landscape with razor sharp musical rapiers.

For my preferences (on the dark and clear side of the treble spectrum), this is a tuning that balances out detail and weight really well, providing the spacious staging and a large sense of air to the upper half of the sound without affecting the carefully constructed neutrality or organic tone. It drenches tracks in upper-frequency microdetail, but does it a self-effacing and almost modest way, pushing the detail out but refusing to shout about it. This can compete on detail with some of the big guns like the Zeus, but without sacrificing the overall tuning to achieve this.

Cymbals are crisp and suitably splashy, the decay feeling long enough to fizzle out with the proper sense of realism rather than dying off in a dull metallic splat. “Crazy Joey” by Joe Satriani has a prominent (and complex) hi-hat rhythm that sits just on top of the guitar and bass, and the C9 highlights super-drummer Marco Minnemann’s exemplary control here, giving enough air to the ringing metal to really drive the track along. Ditto for “In My Pocket” by the same artists, with the C9 providing a crystal clear sonic landscape for the cymbals to swirl around and duel with the numerous guitar harmonics Satriani likes to throw into his music.

“Go” by The Chemical Brothers is the last tester I am throwing into the mix, and again the C9 does well with the driving hi-hat rhythm that drives the track, bringing it clearly into the forefront of the soundscape without overly sharpening or drawing too much emphasis from the rest of the sound. The swirling synth lines that sweep from left to right in the chorus are suitably euphoric, moving lightly through the sound but carrying a sense of substance and definition that catches the ear. In fact, the upper end in this track sounds as good as anything I have heard, keeping pace with the TAEC-driven Campfire line and the Zeus-XR from Empire Ears for my personal preferences. It actually reminds me a little of the effortlessness of the new breed of e-stat tweeters that are started to be used in some high-end IEMs, delivering a sparkly and present treble without any apparent sense of effort or sharpness.

If I had to pick one area that really excels on the C9, I would probably pick the treble – it has a compelling balance of weight, sparkle and air that makes some tracks absolutely sing in the ear, and presents cymbals as well as anything I have in my collection. It can trade detail with the top tier, and is still smooth enough not to bother my treble shy ears. Six drivers? For treble this good, I can see why.

Soundstage, layering and separation

The C9 throws out a stage that is reasonably wide, pushing a little outside of the ears in both directions. It isn’t the broadest I’ve heard, keeping a fairly realistic lateral spread rather than exaggerating the distance at either end. It balances this with a good sense of depth, presenting a stage that varies from slightly oval to almost fully spherical depending on the track. The more intimate nature of the instrument positioning stops the C9 from feeling fully “holographic”, but it does do a very good job of immersing the listener in the middle of a well proportioned “bubble” of music.

Height is also presented well, with the C9 positioning cymbals notably above the bass drum on tracks like “Shelter” by Ray Lamontagne. Both this track and “Trouble” off his debut album show up well, the Stealth Sonics flagship pushing the hard panned drums fully into the left hand side of the stage, and placing Lamontagne dead centre, with the jangling acoustic guitar refrain occupying a space on the far right. Some monitors can tend to push the instruments out too far to each side, stretching the sonic image and presenting it quite flatly in the ear – not so with the C9, which manages to keep most of the width but also the depth and individual placement.

Listening to “Better Man” by Leon Bridges, the cavernous recording venue feels well constructed, with the sax break and faint whistling that come in around the 0:57 mark drifting over the left shoulder of the listener. Overall, imaging is a considerable strength of the C9 (and it’s universal sibling), with the right source and music recording allowing the listener to put each performer in a specific spot in the soundscape and move around between them without any effort.

Separation and layering are also high level, with the C9 having little difficulty presenting busy tracks like “Coming Home” by Sons Of Apollo without any hint of congestion, spreading the thunderous drum fills out across the back of the listener’s skull and layering slice after slice of bass and guitar on top without muddying anything together. The relative speed of the dynamic driver and lighter tuning helps here, allowing all three frequency ranges to keep pace with each other on more frenetic tracks without blurring or smudging the sonic information.

In keeping with the overall tuning, there isn’t a huge amount of black space between each sonic layer or specific instrument, but they are all clearly separate and identifiable, running a nice balancing act between clarity and overall cohesion of the sonic image. This isn’t a surgeons scalpel dissecting your favourite tunes into its component organs, but at the same time it isn’t a sledgehammer smashing bits and pieces of distinct sonic info together either. It treads a good line between analytical and musical, in keeping with the rest of the tuning choices.

Power requirements and synergy

The C9 isn’t a hugely hungry IEM, getting loud enough off my lowly Pocophone F1 to make my eardrums uncomfortable. Conversely, the dynamic driver does seem to respond well to slightly more power being fed through it, thickening up slightly when fed with an amp like the ALO Continental V5, or out of my Cayin N6ii on higher gain. I have volume matched just to make sure, and it does seem to my ears that you will get a slightly more authoritative bass presence with a higher power output source. So, while amping is definitely not a necessity, the C9 does response well if you have some spare juice on tap.

With regards to synergy, the neutral-natural sort of tuning works best for me with a similarly warm source. The Cayin N6ii (with stock A01 motherboard) is a good example, the slightly thicker low end and analogue sounding warmth of the Cayin amp section bringing the best out of the 10m DD and giving the C9 a nicely organic tone, and using the full extent of the C9 resolution and imaging prowess. The Stealth Sonics flagship actually plays very nicely with the N5IIS from the Cayin range as well, again matching nicely with the tonality of the player to accentuate the best aspects of the C9 presentation.

Pairing with a colder or more analytical source like the Fiio M11, the C9 gives more of a true reference type of sound, feeling a little more clinical in execution. For me, this robs the sound of some of its listenability, losing a little anima in exchange for a colder and slightly less intimate tone. If you are looking for a more neutral sort of IEM then pairing the C9 with more analytical or colder tuned DAPs will yield pretty good results for you – the C9 does seem pretty transparent to the source it is being played from.

As it doesn’t need much power, I have also run the C9 off things like the Shanling M0 and Sony Nw-A45. Neither DAP makes the most of the resolution on tap, but the M0 sounds surprisingly energetic, the miniature Shanling powerhouse driving the C9 pretty well. The underpowered amp section of the A45 doesn’t yield quite such good results, giving a nice warmth to the sound but just leaving it feeling a little lifeless in comparison to the other sources mentioned above.

Comparisons

Stealth Sonics U9 – (Hybrid, 1xDD 8xBA, c. $1099)

The first comparison will be a simple one, as it is with the universal version of the C9, handily called the U9. The U9 is identical in terms of driver count, utilising the majority of the technology from the custom version and swapping out the “Klarity Valve” tech for a removable faceplate which serves as a damper for the bass.

The shells are made of a light rubberised material (Stealth Kompozit), and have a shape that is reminiscent of the Hibiki from Shozy/AAW, with a whiff of the Noble universal shell design thrown in for good measure. Compared to the C9, the U9 build feels lighter, with a short nozzle leading to a fairly shallow fit in the ear compared to the full custom effect.

This will be a short comparison as the sound is very similar between both models, as you would expect. The main differentiator is the bass – the C9 simply has more of it. Whether this is to do with the full custom fit or something else I can’t say for certain, but the C9 definitely sounds the fuller and warmer of these two models. There is more sub and mid bass quantity, with low end sounds filling out a little more in terms of thickness, and providing more overall warmth to the rest of the sound signature as a result. Both drivers are identical, so in terms of technical performance there isn’t any appreciable difference, but the tuning does make itself felt in other areas.

Moving up to the mids, the C9 sounds weightier and more organic, due to the extra bass fundamentals. The difference isn’t huge, but it is definitely noticeable, with the U9 coming through as colder and more analytical in tone. Due to the lower volume of bass, the U9 actually comes across as slightly crisper and clearer in the mids, with additional space around each of the notes on a plucked guitar chord or singer’s voice, trading warmth and substance for a more surgical rendition of the underlying detail. If you listen, the C9 doesn’t actually have less clarity, being able to resolve passages in a similar fashion to its universal sibling, but the additional warm air around the instrumentation brings everything a little closer together, and pushes the detail retrieval a little further into the background of the sound. These are both detailed IEMs, but the U9 is more obvious about how it presents it, so if you prefer cold and clean to natural sound, the U9 leans more that way.

Treble is pretty much identical on both models, with the C9 just pulling in front in terms of audible detailing here due to the increased isolation making it easier to hear the really fine transients and harmonics in some of my favourite recordings. Both are top tier performers in this regard.

Isolation and fit are won hands down by the C9, as you would expect. Separation and layering are again slightly more pronounced on the U9, but only in the sense that the notes are thinner so it is easier to pick out the different strands of music – the C9 presents a picture that is just as delineated, but sounds fuller and warmer so doesn’t shout about it quite as much.

Overall, the C9 and U9 are two sides of the same coin. The C9 adds a little bass quantity and warmth to the sound, painting a more solid feeling and organic sounding image in the ear. The U9 leans more heavily towards analysis, losing some of the addictive substance from the low and mid ranges and trading it for a more etched presentation that pushes detail up in to the ear in a more obvious fashion.

For my preference, the more organic and substantial sound of the C9 wins out for me, as I think it is the “better” of the two in terms of overall sound reproduction, but fans of a more clinical or colder sounding monitor would probably opt for the U9.

Wavaya Quadra – (CIEM, 4xBA – c. $600)

Comparing the C9 to the Quadra from Wavaya is probably a little bit of an unfair comparison, given that the Quadra comes in at less than 50% the cost of the Stealth Sonics IEM and only contains 4 BA driver. I am including it here for two reasons: to show how well the Quadra competes in the next price bracket up, and also because these are the only two customs I currently have that are made from exactly the same set of ear impressions, as the same digital STL files were used as the basis for both monitors.

Starting with packaging and accessories, the load out is fairly similar, with a case, some travel adapters and a small cleaning tool all coming with both. Wavaya diverge slightly by including some stainless steel ear cleaning tools in their box (that look like medaeval torture implements), which is definitely a first for me. In terms of cases, both IEMs provide a nice personalised case, but the Stealth Sonics effort isn’t quite the same quality as the all-leather Wavaya case. There is a lot more room inside the Stealth Sonics case to add additional cables or a small DAP, but it lacks the solidity and simple winding mechanism of the Wavaya case, which I feel keeps the IEMs more securely locked in place. The stock cable on the Quadra was a Plastics One cable, but they now offer the option to upgrade to a Linum BaX cable, which its definitely worth doing, and brings it into line quality wise with the Stealth Sonics package. Overall, honours pretty even (apart from the case).

In terms of fit, the IEM shells aren’t identical (even though they are both taken from the same STL file). The designers at Wavaya have opted for a slightly longer nozzle, and the Stealth Sonics team have a slightly shortened nozzle but a marginally thicker stem. In use, both provide an excellent fit, with the thicker nozzle of the Stealth Sonics allowing full seal to be maintained while chewing, where the Wavaya occasionally breaks seal. Both are comfortable to wear for extended periods, but the porcelain shells of the Quadra feel less “obvious” in the ear after a while, seeming to melt away in use in comparison to the very comfy but always “present” feel of wearing the Stealth Sonics shells. My ears also seems to produce less wax on the IEM shells using the Quadra.

Sound wise, both present a musical take on balanced – the C9 gives a slightly warmer and meatier tone to the music due to the DD underpinning everything. There is more low end on display with the C9, which has a thicker and more present midbass than the relatively cleaner Quadra. Sub bass is also more powerful with the dynamic driver, giving a little more thrum to my usual test tracks from artists like Emile Sande. There is a difference in texture as well – both IEMs produce a beautifully detailed bass, but the C9 is a fraction slower in its decay, letting the bass drum impacts linger a little longer in the ear compared to the snappiness of the all-BA Quadra. The C9 obviously shifts a little more air into the ear with the 10mm driver, so for sheer physicality, the C9 has an advantage here, presenting drums with more physical impact and slam. Texture is a wash, with the Quadra presenting cleaner and leaner with bags of fine detail coming to the fore, but lacking the physical substance that the C9 provides to the edges of notes.

Moving up to the midrange, vocals are pushed further forward on the Quadra, which presents a more compact and intimate staging to the listener. Singers are a little less thick, with the trailing edge of notes feeling a little sharper on the ear from the Wavaya model. It balances this out with some cooler air between the instruments, allowing for a little more breathing room and sense of crispness on stringed instrumentation in particular compared to the Stealth Sonics flagship. Detail level is similar, with the crisper notes of the Quadra giving a more immediate impression of clarity. The C9 does pack in a LOT of detailing into the main mix, but it resolves in a less immediate way, matching (and occasionally exceeding) the Quadra in raw resolution, but filling the notes out a little more so it doesn’t always scream for attention. The emotional sense of rawness that the Quadra can bring to some vocal deliveries is replaced by a sweeter and more lush tone with the C9.

Treble is nicely extended on both monitors, but the C9 has the nominal edge with a rated extension of 40kHz. This is done so the treble will stay relatively flat past the usual hearing threshold, and it does provide to have a slightly stronger pedigree in the really high reaches of some of my tracks. Both monitors share a similar presentation, going for a thicker and more bodied presentation rather than cut-glass etching, so there is nothing to separate them from a preference point for me there.

The additional extension on the C9 aids its imaging, which is top-tier, and presents a stage that is a little further back from the listener but much easier to locate each individual instrument on in terms of position relative to the listener, giving a more solid stage image than the Quadra. The Wavaya model counters with a slightly cleaner sense of separation, using the slightly larger space between each instrument to keep things more obviously separated and layered. Again, the C9 is a top tier operator here, but the relative warmth of the tuning makes it a little less apparent in direct comparison to the cleaner Quadra presentation.

In terms of driving power, both IEMs require roughly similar amounts of power to drive from my DAP collection, so in practical terms don’t have much to split them. They are also both dead silent – Stealth Sonics are another IEM firm that have their roots firmly in the stage musician market, and the non-fatiguing and hissless presentations of both is refreshing.

Overall, the C9 is the fuller and more romantic sounding IEM of the two, giving a thicker sound and more physical impact. In terms of technicality the Quadra is able to hang with an IEM over twice its price, only falling behind in terms of imaging but matching the C9 in terms of resolution. The Quadra is slightly sharper in tone and presentation, giving a more energetic and musical “vibe” to things, in comparison to the slightly more laid back and smooth presentation the C9 puts out. The C9 is a proper flagship-level IEM, and on a purely technical basis it pulls slightly ahead of the Quadra. If you are looking for something to use in the studio or for critical listening without sacrificing bass or a sense of musicality, the C9 will probably be the better choice out of the two. If you are looking for a monitor for stage use or just for extracting maximum emotion without sacrificing on technical capability, the more “in your face” style and tuning of the Quadra make this an interesting offering at around half the price.

Campfire Audio Solaris – (Universal, 1xDD 3xBA – c. $1499)

The Solaris is the current hybrid flagship from Campfire Audio, sporting one 10mm ADLC dynamic driver and three balanced armatures in a 2-crossover design. The team at Campfire Audio are quite comfortable to trumpet various unique technology they use to achieve their house sound, giving the C9 a run for its money in the proprietary technology stakes with the diamond-coated dynamic driver and the Tuned Acoustic Expansion Chamber (known catchily as TAEC) which they use to run the treble frequencies through a tubeless resonator to give a wider sense of air and space.

The Solaris actually packs a lot less drivers into the shell, running one ADLC dynamic driver (the same one used in their Atlas model), a single full range BA with emphasis on the mids and a dual armature tweeter setup hooked up to their TAEC technology. This contributes to a tuning that is fairly similar in nature to the C9, with the Solaris feeling just a little fuller and more rounded than the Stealth Sonics model. Extension is similar on both models, digging deep into the recesses of sub-bass without any noticeable roll-off. In terms of layering and detail retrieval, the C9 driver is good and tight, but can’t quite match the excellent technicalities of the Campfire ADLC model, just being shaded in overall texture and fine detail. Neither model is suited for extreme bass heads, but the Solaris leans just a shade heavier and more substantial in the low bass frequencies, giving more of a “floor standing speaker” style effect when listening to bassy tracks.

Mids are a little thinner on the C9, pulling the vocals slightly further back on the stage compared to the more intimate presentation of the Solaris in terms of both stage position for the vocals and overall note size. The C9 is similar in tone, tending to feel just a little more neutral or cooler (but not as different as its U9 sibling). The C9 gives a little more emphasis to the edge of notes, leading to perception of a (relatively) sharper response that carries a little more emphasis on detail in comparison to the more laid-back and natural sound of the Solaris. This does come at the cost of a little of the Solaris’ organic timbre and tone, with the C9 sounding more “reference” or neutral in comparison to the more coloured presentation of the Campfire model.

Treble is sharper and more emphasised on the C9, again feeling a little thinner than the Solaris in both both tone and overall weight. Both IEMs provide the listener with a solid and holographic image, with neither IEM pulling ahead significantly here. Stage size feels similar on both, with the Solaris providing the bigger picture with greater note size and intimacy and the C9 taking a more laid back and widescreen approach.

In terms of driving ability, the Solaris is the easier IEM to drive, with the C9 requiring a little more power to get to the same listening volume on my usual gear. C9 is a lot less picky with source OI, staying pretty stable with differing output impedance and offering not much in the way of hiss with any of my current sources.

In terms of the overall package, both IEMs are fairly similar. Campfire products are renowned for having a good unboxing and accessory experience, and the Solaris is no different, packing in a real leather case, three different types of tips, a CA pin and cleaning tool and ne of their “SuperLitz” SPC upgrade cables as standard. The accessory loadout and cables on the Stealth Sonics are both good for this price bracket, but not quite up to the full after-market experience of the ALO SuperLitz cable and Campfire leather case in terms of looks.

Overall, these two IEMs are more similar than they are different. They possess similar imaging prowess, but diverge quite significantly in terms of tuning, with the C9 treading a more reference or neutral pathway that errs more towards a cooler and thinner tonality, with less physical dimension to the individual notes (both in terms of weight and overall “roundness”). As a result, while I am impressed with the technical prowess of the Stealth Sonics model, I find the Solaris can be a little more immersive when you just want to lose yourself in a track. It has more musicality than its universal sibling the U9, so competes on a more level playing field with the Solaris there, but the difference between them really boils down to the listener’s preference for analytical or more “musical” tunings. If you are looking for reference, the C9 is a better recommendation – if you lean more towards the richer end of the sonic spectrum, the Solaris is the way to go. For my personal preferences, the Solaris is still the best IEM I have personally heard, but the C9 reminds me of it in a lot of ways, which is definitely a good compliment.

Dunu DK-4001 – (Universal, 1xDD 4xBA – c. $899)

This may seem like an unusual comparison, given that the DK-4001 is the flagship model in the current Dunu lineup but has a pricetag just under 2/3 of the C9’s cost, and is also a universal IEM rather than a custom. The reason I have included this is that the Dunu shares a very similar style of tuning and technical ability, so felt like a good comparison, as the overall sound is close enough to the C9 on enough levels to make this an interesting A/B for me.

Starting with the bass, the C9 has slightly more midbass presence, but only marginally. Neither are full on basshead IEMs, but the C9 just has a little more oomph down low compared to the Dunu. This gives the C9 a slightly warmer overall tonality in comparison to the colder and more clinical DK-4001. Sub-bass is similar, with the emphasis probably tilting more towards the Dunu model, but by a smaller margin. Punch is similar on both, with the Stealth Sonics DD keeping pace pretty well with the larger (12.6mm) beryllium coated DD used in the Dunu IEM.

In the midrange, the DK-4001 has a more emphasised vocal range compared to the more neutrally positioned C9, with the upper-mid peak of the Dunu helping pushing vocals slightly more to the forefront. The C9 counters with a richer and meatier presentation of both male and female vocals, but again isn’t a million miles away from the Dunu. Guitars and stringed instruments are presented excellently on both, with both carrying enough edge and crunch to really kick tracks like “World On Fire” by Slash into a higher gear. The Dunu presents notes with a slightly sharper edge, but like the vocals also with a lighter and less dense body. The C9 feels fatter and denser, but without losing anything in terms of resolution or crispness. The best analogy I can think of here is between a rapier (the Dunu) and a katana (Stealth Sonics). Both sharp enough to cut through the chunkiest of tracks with ease, but one just carrying a little more weight behind it than the other.

With regards to staging, sound feels deeper on the C9, with a better sense of position across the z-axis to give a more spherical stage. In comparison, the 4001 feels a little wider and flatter, presenting as a more oval stage. Spatial cues are much more apparent on the Stealth Sonics model, giving the music a more three-dimensional mental image, painting instruments with a more precise location on the stage. This also has the effect of making the separation and layering a little easier to pick out on the C9, due to the increased sense of depth. The 4001 is still a highly capable IEM in this area, but the C9 is definitely a class above here for me.

In terms of overall shape of the tuning, the C9 has a smoother and flatter frequency response across the spectrum, with instruments sounding a shade fuller and more natural, and no obvious spikes or areas of emphasis. Detail is more audibly emphasised in the midrange on the 4001, but the C9 actually resolves as much (if not more) as the Dunu. Taking one of my favourite testers (“Palladio” by Escala), the subtle click in the intro bars as a musical stand is adjusted in the background is heard as clear as day on 4001. The C9 still captures this piece of sonic background info, but it isn’t pushed as far forward into the consciousness of the listener, settling a little further into the audio backdrop. Regarding tonality, sounds feels sharper and colder on the 4001, with a more clinical tone – the C9 is warmer and more organic sounding in direct comparison.

Driving requirements are roughly equivalent across both IEMs, with both responding to more voltage with a similar result on the bass. Neither are particularly hard to drive.

Accessories and packaging is good with the C9, but the Dunu takes the lead in this aspect with a more ostentatious unboxing and an excellent array of accessories (including a very nice leather case). The real differentiator between the two is the new Dunu cable, however – it sports their new interchangeable plug design at the end, and comes as standard with 2.5mm/3.5mm/3.5mm “pro” balanced/4.4mm connectors in the package. This is a killer add-on for something in the Dunu’s price bracket, and pushes the Chinese flagship ahead of the Stealth Sonics model here.

Overall, both IEMs play in the higher end of the scale when it comes to both resolution and overall sound quality, with a similar but not identical approach. The C9 has a slight technical edge with better imaging and staging, with similar levels of detail to the Dunu hybrid. It also manages to eke a little more bass out of the smaller DD driver without sacrificing any control – this, allied to the slightly more organic and natural feel to the sound mean that if I had to choose one IEM, I would probably lean towards the C9 for my particular preferences. That said, the Dunu keeps pace on multiple fronts admirably, which is pretty impressive for an IEM that is less than 2/3 of the price. If you are looking for an IEM with a thinner but more emphasised midrange tonality and prefer your sound on the colder or more neutral side of the street, the Dunu would be my suggestion – if you are a fan of warmer or more organic tunings (without losing any detail) or a holographic stage is more important to you, the C9 is an easy recommendation here.

Overall thoughts

The current high-end IEM scene is an interesting market, with some manufacturers pushing the boat out with various new technologies and eye-watering pricetags of $3500 and upwards for their “endgame” product. The C9 has a price that is certainly high enough to qualify ($1499), but now occupying the curious “middle ground” with IEMs like the Campfire Audio Solaris and new Lime Ears and Custom Art flagships.

Is the C9 a worthy contender to be considered at the top table of current IEM achievement ? For me, the answer is a resounding yes. There is a beautiful mix of technical capability and easy musicality that marks the C9 out as something pretty special to listen to. It won’t blow you away at first listen, with a signature that cleaves close to neutral but adds just enough warmth and verve to avoid sounding sterile. It is the sort of tuning that creeps up on you over time, impressing you with hidden snippets of resolution, or beautiful splashes of vibrant treble and textured bass. It impresses also with its crystal clear imaging, scaling very well with higher end gear to give a very solid sonic picture of what you are listening to, creating an immersive stage to drift into with your favourite tracks.

Being honest, it won’t win the battle as the most technically gifted monitor in the $1k+ bracket, but it does pack enough resolution to give most TOTL IEMs a good run for their money. It won’t beat your brains in with bass, but it can get your foot tapping incessantly when you feed it the right track. Apart from treble, there isn’t one standout area where it is truly exceptional, but in counterpoint, there are no areas where it is anything less than very good. It is simply a great sounding IEM. If you are looking for something precise and reference-grade but still love your music to sound musical, then the C9 is a strong choice.