Stealth Sonics U9: bass plus refinement

Pros: Precise accurate bass tuning, tasteful bass uptick, timbral accuracy all around, good stage width and height, high quality stock cable, includes microphone cable (flexibility+), light-weight

Cons: Upper mids can be a bit aggressive, aggressive elements of presentation reduce perceived stage, case not very usable from a travel perspective, does not look or feel like a $1099 IEM

List Price: $1099

Product Website: Stealth Sonics U9

Stealth Sonics U9

Rating Disclaimer: ratings are subjective. Audio quality and value do not mean the same thing across all prices. A headphone with a 5 rating on audio at $5 does not have equivalent sound quality as a 5 rating at $500. Likewise, value at $5 is not the same as value at $5000 dollars.


Stealth Sonics caught my partner in blogging’s eye at CanJam London 2018 and he has since done some initial impressions of the U9 and U4. A review of the C9 is coming, which is where I think this might get really interesting. Jackpot77 set up this review tour with Stealth Sonics with the ability of the participants to get their choice among the 3 universal units for diligently doing their review work. We had two weeks, which is always a challenge when reviewing, especially since I’ve got other stuff hot on the stove cooking in review land.

A little history on Stealth Sonics:

  • Started in Singapore, by a group of audiologists from a company called MyEar and a NASA engineer (later investment banker)
  • Customs made in Germany; universals made in Singapore

Audio quality

First things first, the bass on these doesn’t really sound like hybrid bass with the faceplate on (or off for that matter). Hybrids tend to have quite elevated bass and the sound here is more neutral bass (still tiny bit of lift). With regards to faceplate on or off as an option, I found that it made less difference on the U9 than on the U2 or U4. On the U2 and U4, opening the plate mitigated some negative sound characteristics in the bass department, whereas on the U9, the bass is tuned better than the U2 or U4 at baseline. With all the faceplate changes, a caveat needs to be applied, as changing faceplates is slow and requires some good organisation and dexterity. The little screws are easy to lose and the Allen wrench is tiny and fiddly.

If I were designing these, I would have done the faceplates differently:

  1. I would allow sound to be changed without removing the faceplate.
    1. This could be done by having the decorative turbine be elevated and used as a functional aluminum knob linked to a gear mechanism that would cover or uncover the holes.
  2. I would make the faceplate out of actual carbon fibre, not a fake printed pattern.
  3. I wouldn’t want the faceplates to be commonly user removable, so I would recess the screws (and not have them be vents) and use a small Torx screw head.

For my comparative work below, I used faceplate on.

Source Headphone Cable SE/Balanced Gain Volume ~SPL
Questyle QP2R (High Bias) Stealth Sonics U9 Stock SE Low 94 78.3
Questyle QP2R (High Bias) Stealth Sonics U9 Effect Audio Ares II+ Balanced Low 84 78.2
Questyle QP2R (High Bias) UERR Stock SE Medium 88 78.3
Questyle QP2R (High Bias) WAVAYA Octa Effect Audio Ares II+ Balanced Low 79 78.2

Stealth Sonics U9 ($1099) vs. UERR ($999)

Build and feature comparison

The UERR is a custom and comes with a nice press-fit puck case. I really like the UERR case even though I have to take it out in airports. I don’t like the case included with the Stealth Sonics universal line-up even a little bit. I think most people will never travel with this giant case and the included pouch is too flimsy. New versions of the UERR come with a cable system that is an evolution of Estron’s T2 connector, IPX. It’s a damn fine connector. The UERR cable on new ones is a Linum BaX cable. The quality of the included cables is a push. My version has the old black plastic UE 2-pin cable. Overall build quality and quality of accessories definitely goes to the UERR.


For the comparison, I used stock cables for both out of the Questyle QP2R volume matched using an SPL metre and a custom made (read jury-rigged coupler).

Bass extension and body is excellent on the U9 when testing out Yosi Horikawa – Wandering (16/44, binaural). I wouldn’t be surprised if the bass driver is the same as the U2 as the texture is very similar, just without the sheer amplitude. The attack and decay of the U9 bass is dead on in the deep sub-bass rumblings of this track. Individual elements of the track are well resolved in space. There is a bit of extra energy in the treble. The UERR represents most of the same bass tone but doesn’t get the feel and rumble that you get with the U9. Timbre-wise, these are very similar. Both do an excellent job of resolving the spatial elements of the track. The U9 has a bit more lower treble energy, which can make some elements sound a touch overexposed.

Macy Gray’s voice on The Heart (24/192, binaural) is a touch uneven on the UERR, there is a little bit more nasal quality while also being dry from how her voice decays, it’s stuffy but short sounding. The U9 is a touch more forward on Macy’s vocal, and sounds cleaner and clearer. It’s better. I’m also getting more clear resolution of the tight and light treble tappings on the cymbals. The U9 are basically a slightly more fun neutral reference with a bit of extra sub-bass and a bit of extra treble where the cymbals crash, which is right on my kind of preference. Macy’s voice is soulful and the tuning of the rest of the spectrum is natural and gorgeous. The UERR feels a bit brittle, while also feeling a bit congested on Macy.

Billy Cobham – Quadrant 4 (DSD64) is a coked-up jazz-rock hybrid with killer sonic variety. The UERR keeps up okay, but there is a bit of blending in the cymbals, a kind of sustained shimmer with little tings of definition. The treble transients on the U9 are tighter and sharper. Whether this is more accurate is a fair question, but it sounds quite detailed. Big cymbal crashes have excellent timbre and amplitude. The amplitude of big cymbal crashes is less pronounced on the UERR, which is actually more positionally accurate than the U9, but less exciting. The bass groove is really nicely textured on these. Speedwise, the U9 are a touch faster across the spectrum.

Stealth Sonics U9 ($1099) vs. WAVAYA Octa ($1590)

Build and feature comparison

Both headphones come with reasonable accessories. The Octa comes with a swish Florentine leather case that is on the small side but well designed for the included cable; it doesn’t accommodate after-market cables well. The U9 comes with a faux leather zip-case that is way too large and doesn’t quite do it for me. The Tria case is objectively higher quality. The case that is more likely to meet folks needs in general is the WAVAYA case, as it is a more usable size for throwing in bags. Real leather also wears better than fake leather. Fake leather like that on the Stealth Sonics case peels and cracks in unappealing ways when tossed around too much due to it basically being a thin laminate material. The Stealth Sonics line-up comes standard with a recessed 2-pin cable, while the WAVAYA Octa can be had with a T2 Linum SuperBaX or a 2-pin Linum SuperBaX. I didn’t have the SuperBaX when doing this comparison, so used the Effect Audio Ares II+ for this comparison. I really like the SuperBaX cable, which has superior ergonomics to any cable I’ve used.


For my comparison with the WAVAYA Octa I had each headphone on the same cable, the Effect Audio Ares II+. The effect of the Ares II+ on the WAVAYA was to improve the bass volume and definition on the WAVAYA Octa while boosting clarity a bit, so I expect a similar effect on the U9.

The violins are deliciously rendered on Regina Spektor – Field Below (16/44) and Regina’s voice sounds nice, but a bit forward of neutral on the U9. Piano has great body and texture on the U9 but can be a bit hard and forward. A little more restraint would be good here. It’s overall a very solid performance, especially at $1099. There is a touch of shoutyness to Regina’s voice on the U9. Piano is more delicate and refined on the Octa with just the right amount of reverb. Regina’s voice is still forward but doesn’t get as shouty (still some shouty, probably just track mastering). There is more ambiance to Regina’s voice with the more texture extending out into her overtones and more nuance in the vocals. The refinement of the Octa is superior on Regina’s vocals, giving a more lifelike presentation. The stage on the Octa has more depth and instruments have a more natural presentation in space with better separation than the U9.

Yes – Sound Chaser (24-96, Steven Wilson remaster) is a speed test track for me. The U9 keep up with good impact. All the little treble tings, cymbal shimmers and percussive impacts are rendered very nicely. The soundscape is busy, but individual instruments can be picked out, with a little effort in their own space. Stage depth is not as good as the WAVAYA Octa, but it is still good. Instrument separation is really excellent on the Octa, with different elements of treble finding what feels like their true place and character in the right layer more consistently. Individual vocalists are easier to pick out on the Octa. Tone on drums and bass is accurate on the Octa, but impact is better on the U9 due to having a dynamic driver built in. If the Octa was using a dynamic driver here instead a balanced armature for the lows I think it would give the same kind of effect—assuming the right dynamic driver were used.

Outkast – Sorry Ms. Jackson (16/44) is a surprisingly layered and textured track. The treble on the WAVAYA Octa is superlative with layering, definition, and a sound that sounds ‘just right.’ There is a fundamental correctness to the treble presentation. It pulls out every little detail while still having a good decay and tonal accuracy. The treble just sounds real. On this track the WAVAYA exhibits a bit extra snap in the 3kHz – 4kHz range on the snare job, thrusting the impact hard forward. Bass is tight and controlled while still having some groove on it. Vocals are more forward on the U9 and a touch smoother and sweeter. Stage width and depth are a little bit smaller than the WAVAYA. The bass on the U9 has a bit more feeling, though the frequency response capabilities are pretty equal. The Octa has more roll off at the lowest end of the sub-bass range, which makes me inclined to like the sub-bass presentation of the U9 a little more for this hip-hop track. I like the less forward mids a touch more on the Octa.

I’m getting great texture in the treble on Blue Oyster Cult – (Don’t Fear) The Reaper (DSD64). The guiro comes through with some nice firm texture without being pushed too forward. Cowbell is a touch more forward than usual but not overexposed. There is good width, getting outside the ears and good height presentation. Because of a tendency toward forward elements in the signature, the depth is fairly average. The guitar solo in the middle of the track is silken in its transitions with excellent decay characteristics and good speed on the attack. Cymbals have good definition and remain back of the vocals (this is a good thing). The overall signature of the Octa is less forward. Guiro and cowbell sit at similar depth but vocals are less forward. The sound has a bit more natural delineation of instruments. There is effortless separation, which causes me to notice the separation of individual vocalists in vocal harmonies more than I have before. Bass on the Octa is present, but not emphasized at the lower end like the U9. I personally prefer the U9 bass, but this is because extra sub-bass energy is always welcome and while a balanced armature can replicate the tone and timbre of deep bass well, it can’t match the palpable power that the dynamic driver led setup of the U9 can do. Treble on both is excellent, but the Octa is more refined and effortless which gives a better stage and a more real feel in vocals and instruments. The timing of the Octa treble just has more sonic information.

Yoni Wolf’s vocal comes out with that sweet nasally quality that I’ve come to expect and cherish on Why? – Strawberries (16/44) with the WAVAYA Octa. There is excellent separation and attack in the numerous treble and upper mids percussion instruments with natural decay all around. You can almost hear the individual grains in the maracas. Detail plus on the WAVAYA Octa. The synth bass and claps are a bit bigger on the U9. Yoni’s voice is sweeter and a bit less nasally than the WAVAYA, which means there is a touch of colour in the U9 signature because Yoni should sound a bit more nasal. The stage width is a bit less on the U9 than the Octa. Maracas are rendered just as precisely and decisively on the U9 as on the WAVAYA, which is impressive given the fact that the Octa uses 2 pairs of electrostatic tweeters.

Overall, the WAVAYA Octa was the superior IEM, but the gap is less than the $500 price difference would indicate—diminishing returns and all that. The WAVAYA wins on separation, refinement and the way the mids and treble just sound more real. The U9 has better bass and impact because that’s what dynamic drivers do best.

Usability: Form & Function

The whole universal series has the same physical characteristics, so I’ll point you to the U4 review for more on that. I’ve just dropped the ergonomics section for reference in here.


I find the shells a bit on the large side, but comfortable. I imagine that the shell is identical for the U2, U4, and U9 which means that they all have a shell designed to fit 9 drivers. The part of the shell that settles into the concha of the ear doesn’t have any contouring of the crus of helix area that would make people mistake it for pseudo-custom shapes like the those found on iBasso IEMs or Kinera units. The shells take into account the average shape of the tragus, antitragus and intertragic notch, which means that the fit is secure and comfortable at the most important contact point on the ear.

Ear structure

The soft touch coating is indeed nice to the ear. I’d guess that it is hydrophobic, but I haven’t tested them out in a really sweaty environment.

The cables are very ergonomic, and also strongly resemble the Hibiki cable, which is made by Null Sound (the cable division of AAW). They forgo memory wire in favour of preformed plastic sheathing (heat shrink, essentially). This is my favourite ear hook method on cables. The tops of my ears are sensitive—actually my whole ear is sensitive—which results in irritation when I have heavy cables on my ears or memory wire. I have no problems at all on these cables. For glasses wearers the low profile of the cable should ensure compatibility with your glasses.


Specifications U9
Price $1099
Driver type Nine-driver, hybrid design (1DD, 8 BA)

1DD = low

2BA = mid

2BA = high

4BA = super high

Frequency response 18Hz – 40kHz
Impedance 16Ω @ 1kHz
Sensitivity 108dB @ 1mW
THD ≤1% @ 1kHz
Isolation -26dB
Construction (specific) 4-way crossover

3 bore

Silver accents

Construction (shared) Polymer body with soft-touch coating (Stealth Kompozit™), removable faceplate (screw holes act as vents, allowing more driver incursion leading to greater bass presence), recessed 2-pin cable attachment
Accessories Extra large carrying case, polishing cloth, airline adaptor, Allen key for faceplate removal, 3.5mm to 6.3mm adaptor, single flange silicone tips (S/M/L), biflange silicone tips (M), foam tips (S/M/L), good quality double twist 3.5mm terminated cable with preformed heat shrink earguides (no mic), double twist 3.5mm terminated cable with preformed heat shrink earguides (with mic)
Warranty 1 year


The Stealth Sonics U2, U4 and U9 were provided as loaners as part of a review tour. In return for reviewing all three I will be given my choice of universal Stealth Sonics IEM—I’m choosing the U9. All thoughts in this review are my personal opinion.


The U9 is absolutely stellar at its price, it isn’t a neutral reference, but it leans in that direction within its fun signature. It has a little bit of extra bass, but it’s tasteful and extended. The treble is detailed and the stage is excellent. Mids play well too. I’d reach for these more often than the UERR as my new reference at around $1000.

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