Stealth Sonics U2: soul-shaking bass

Pros: Big smooth bass (faceplate on), big rumbling sub-bass murder by death machine (faceplate off), smooth mids (faceplate on), mids balanced between smooth and well-defined (faceplate off), high quality stock cable, includes microphone cable (flexibility+), light-weight, ability to alter bass tuning (removable faceplate)

Cons: Sounds substantially better in its less attractive faceplate off configuration—why you do that Stealth Sonics!?, aggressive signature, detail lost to smoothness (faceplate on), all plastic construction, no balanced cables on offer (yet)

List Price: $249

Product Website: Stealth Sonics U2

Stealth Sonics U2

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 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

So right off, I’m going to say, there is no way in hell I think these need a faceplate removed to give them more bass volume. Lucky for me, that isn’t mostly what happens on these. When I remove the faceplate, I feel like I’m getting a bit better overall definition of bass in the sub-bass region with a tiny bit more volume. I also get better quality overall sound through improved definition and clarity. It’s unfortunate because it makes them look a whole helluva lot less attractive. I would have had another attractive curved face underneath with a logo or something. The bass is still big though a touch smaller in the midbass with more sub-bass added. With the faceplate off there is greater focus and texture which also makes the mids sound cleaner when listening to all the tracks. This change is actually more in line with what I would expect from increasing venting—that’s all removing the screws is doing really. Better venting means more air moving through and around the driver. These just sound better with the faceplate off. These pitch above their price when the faceplate is off on sound quality and are kind of par with the faceplate on. Comparisons below were done with the faceplate on, as I didn’t have a ton of time with these, and they were the first of three units to review. For the U4 and U9 I’ll test which way I think sounds better before doing comparisons. Unfortunately, I think that most listeners will never even consider taking off the faceplate to change the bass tuning.

With regard to source matching, I didn’t have hiss on anything with the U2, not even the Questyle QP2R. I didn’t try it in balanced mode on the QP2R, so it still could hiss.

Source Headphone Cable SE/Balanced Gain Volume ~SPL
Questyle QP2R (High Bias) Stealth Sonics U2 Stock SE Low 88 78.4
Questyle QP2R (High Bias) Stealth Sonics U4 Stock SE Low 82 78.2
Questyle QP2R (High Bias) WAVAYA Tria Live Cross Lambda Mikumo II SE Low 81 78.2
Questyle QP2R (High Bias) ADV GT3 Stock SE Low 101 78.2

Stealth Sonics U2 ($249) vs. Advanced GT3 ($299) (red treble filter)

Build and feature comparison

The Stealth Sonics U2 comes with a great AAW (null sound) supplied cable that is soft and comfortable, and the shell is light and comfortable in the ear. The shell is made almost entirely of plastic with printed faux carbon pattern and shiny chrome effect collars. The shell is quite large and may not fit smaller ears. The cable connector on the U2 is a recessed 2-pin configuration using standard size 0.78mm pins. I can’t tell if the cable is made of copper, or silver-plated copper. The Stealth Sonics U2 has adjustable bass tuning by removing the faceplate. This feature improves the sound of the U2 substantially when activated, but also reduces the aesthetics and requires using a fiddly Allen wrench to remove tiny easy to lose screws—this should have been executed better. The U2 uses no crossover and hybrid configuration of 1 balanced armature (BA) and 1 dynamic driver (DD).

In comparison, the ADV GT3 has adjustable treble tuning via changeable filters (red = treble, silver = reference, and bass = black). Basically, the GT3 filters adjust lower treble in the 3-4kHz range which changes perception of bass without adjusting the bass frequencies in any meaningful way. The cable on the GT3 is made of silver-plated copper, with a harder housing than the cable on the for the U2—both use preformed heat shrink earguides. The connector on the GT3 is a MMCX connector that appears to be good quality. The IEM is made almost entirely of metal with a plastic collar where the cable attaches. The use of an MMCX connector allows the GT3 to rotate, which means that the insertion angle has some adjustability whereas the fit angle is fixed but at a really good angle that will fit most ear canals.

The U2 comes with a good variety of tips, but so does the GT3. Their tip varieties are basically the same. The U2 comes with a case, but it’s giant while still not being long enough for me to carry my Questyle QP2R in it. The GT3 has an appropriately sized case that is big enough to carry both cables, the headphone and my HiBy R3. It won’t carry the QP2R. Both IEMs come with two cables, with one a microphone cable, but the U2 has a nicer microphone cable.

Overall, I think build and features are close to even. The U2 has better cables, but the GT3 has a more flexible fit and a more useful case. With regards to tuning options, being able to adjust bass will be liked by more than being able to adjust the lower reaches of treble. However, to adjust bass on the U2 you have to turn it ugly by removing the faceplate. I think most aesthetically conscious people will be more into the filter-based tuning system of the GT3.


The U2 has big choogling bass and a forward signature leaning in the w-shaped direction. Dips are in the lower mids, and upper mids (less so here) with big boosts in the midbass and treble. This makes the whole signature feel forward. On Violent Femmes – American Music (16/44) the bass is very much forward, there is lift in the lower treble and upper mids too on the U2. The soundstage width is good, height is quite tall, but the depth just isn’t there because the sound signature is so forward. Clarity is very good when things aren’t too busy. The bass is big but manages to leave other frequencies alone in spite of its dominant size and position in the stage. Gordon Gano’s voice has a nice sweetness to it while maintaining his signature nasal tone. The GT3 has a more restrained bass and an overall colder tone. Vocals, as per the frequency response chart drift back of the treble. The GT3 sounds cleaner and more restrained with a greater sense of depth. The treble percussion has a bit more shake and definition to it, while the U2 has a smoother presentation overall. At the same volume, the U2 sounds louder because everything is forward in the stage. The U2 sound is a bit more on the consumer side, while the GT3 has a more audiophile feel. Stage width is slightly less on the GT3, as is height but the stage feels more spread out because of greater depth to the stage and the effect that heavily boosted bass has on stage perception. The GT3 sound more focused.

Yes – Soundchaser (DSD64) is a speedy track with energy all over the stage. The GT3 keeps up with the speed quite impressively for a single dynamic driver. Drum impacts are textured and well-defined. Vocals feel a bit too far back, which loses some of the sweetness of Jon Anderson’s ethereal vocals. The drums are big, the bass is big, the keyboards are big on the U2. It’s a big friggin’ sound. Drums have a big dynamic feel that leans into the organic/natural tonality, while the GT3 is drier and more analytical. The U2 is big and funky, but a little muddled due to the aggressively forward signature. The U2 isn’t as fast as the GT3, nor as detailed sounding on its face. It is fuller sounding through most of the spectrum with a more engaging sound. Treble sound smoother but less fast, a common trade-off. Bass extension is more audible on the GT3, but the midbass body is less. Overall, I prefer the bass of the GT3, but could use a bit more of it. The U2 is fun but a bit fatiguing for me—I’m someone very much accustomed to a more restrained reference or reference with a bit extra bass or treble (U-shaped) sound. This is not my typical IEM, but I think a lot of people will dig the no holds barred emotive presentation of the U2. I’ll miss the quiet of the spaces in-between, the breathy held notes, the fading twinkle of a high percussive element. I’ll miss delicacy. The U2 is not delicate.

I love Macy Gray, that’s why I use her as a female vocal reference so often. On the GT3, her vocal on The Heart (24-192) sounds a touch distant. The bass presentation is spot on, with good depth and body and that little bit of twang from the rebound of the string. Tonally, the GT3 is doing well here. The guitar plucks are warmer and rounder on the U2. Macy Gray is more forward, but also has a bit of stuffiness coming from the bit of extra energy in the lower mids that is present in her voice. She’s less raspy and airy and more smooth. When the bass comes in its omnipresent, but not as detailed or textured as the GT3. It’s a big and warm bass, not a deep refined bass. The GT3 does this track better.

Overall, I think the GT3 is the better IEM for me, but I think there is a big market for big, warm, forward tuned IEM. It’s a more consumer sound and will have some broad appeal for people who are budding audiophiles. The bass rumble is amazing on the U2, people will also dig that.

Stealth Sonics U2 ($249) vs. WAVAYA Tria ($590)

Build and feature comparison

Both headphones come with reasonable accessories. The Tria 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 U2 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 Tria comes standard with a T2 terminated Linum BaX cable—I did not get this version, instead getting a Plastics One 2-pin cable that I immediately replaced with a better sounding cable. I’d personally recommend that WAVAYA buyers go with the T2 Linum BaX cable.


The bass on Norah Jones – Feelin’ The Same Way (DSD64) feels a bit more in control than on Macy Gray, but this is probably because the track has the bass mastered further back in the stage. The stage width is really good here and the drums are being rendered at a good depth. This track is sounding really excellent on the U2. The WAVAYA Tria Live is more restrained. Bass tone is similar, but bass volume is lower on the WAVAYA with a slightly more textured presentation. Neither is exhibiting as much tonal depth as the GT3 does. Norah Jones’s vocals exhibit a bit more breathiness on the Tria Live. The U2 exhibits a bit more silkiness on Norah’s voice, but also dips in with a bit more sibilance—just a little. The sound of the U2 is really silky smooth on this track, which is quite appealing, but probably a little less photorealistic.

For raw power, Rage Against the Machine – Take The Power Back (16/44) is in the upper echelon. The U2 gets right up in your motherf’in face. Zac De La Roca’s vocal sounds a touch recessed on the U2. As expected, bass has good thump. On this track the texture match is better than on some others. So, these may do better with a more electric style bass than the stand-up acoustic bass on some of the other tracks that have played in these comparisons. At this point in the comparison, I took off the faceplates to see what would happen. Drum got bigger. Bass got more textured and impactful; Zac’s vocals got clearer. The sound is still aggressive, but the tone is more clean and less warm and smooth, which is more to my liking. Even with the faceplate removed and a little air and vibrance breathed into the U2, the Tria Live still has more texture. The Tria Live sounds more balanced with greater stage depth. Some of this could be due to Zac’s vocals being perceived as further back. On this track, stage depth is not huge to begin with, but the Tria Live has an edge.

Spacing on the WAVAYA Tria Live is excellent on Yosi Horikawa – Wandering (16-44, binaural). The synthetic bass has both good volume and good depth. These do an honest job on the bass here. The Tria Live like this modern mastered track. The sound of the three drivers is well integrated on the Tria Live. Switching to the U2, nevermind what I said about bass depth on the Tria. The sub-bass goes way the heck down on the U2 on this track. If you like bass, you are going to dig this, it’s a rumbling sub-bass monster with the faceplate off. It’s bass you feel, not just bass you hear. They should have built the bass tuning function in a more aesthetically pleasing way. The U2 doesn’t have the stage depth of the Tria Live, but holy moley does that bass rumble make up for it.

Stealth Sonics U4 ($499) vs. Stealth Sonics U2 ($249)


Yosi Horikawa – Wandering (16/44, binaural) has a big synth bass drop that goes way down into the sub-bass with loads of texture. The U2 sounds incredible in the bass department here with no shortage of full extension expression. Width and height are good in the stage, but the forward signature of the U2 compresses stage depth and magnifies the scale of instruments. The magnified scale of the instruments reduces instrument separation. It’s a bit of a wall of sound effect. Immediately, the U4 has notably more stage depth, which allows a bit more balance in the presentation. The bass presentation is smaller and less in your face, but with a bit less depth. The improved instrument separation is excellent, but I’m missing the absolute expression of the sub-bass some, as the U2 is so palpable. The U4 has this bass there, but because of how our hearing works, with more sensitivity to midbass, the midbass swallows up some of my detection of the sub-bass.

The U4 slightly thickens Tori Amos’s vocals on Silent All These Years (24-96 vinyl rip). This is probably due to the midbass/lower-mid emphasis of the U4 frequency response curve. This kind of emphasis tends to make female vocals a bit more congested sounding when they aren’t doing high notes. Bass strokes from the cello occupy a large amount of space in the stage due to this emphasis also, which is more space than they should occupy. A touch of haze goes over the piano also from this. Some would call this warmth, I don’t. Tori’s vocals are cleaner on the U2 without that added congestion. The mids are clearer in general (Crinacle’s brilliant graphs show this difference on the frequency response, U2 and U4). This track has relatively little bass, which I find means that the U2 sounds better because of how its lower mids and midbass are tuned.

Bob Dylan’s vocal on Stuck Inside of Mobile with the Memphis Blues Again (DSD64) is on level with the giant bass on the U2. On this track, the absence of a cross-over on the U2 is also noticeable as the bass note strums don’t come out completely clean and warm up the sonic soundscape when there is a lot of action happening in the mids, as there is here with guitar and bass guitar occupying similar space in the stage. With the U4, Bob Dylan’s voice gets a similar treatment to Tori Amos’s voice with added warmth that makes Bob sound a bit congested. The treble spike on the U4 rising between 5kHz and 6.5kHz lends a nice bit of clarity to the upper mids which helps the guitars get a bit more definition. The guitars still don’t escape the space of the bass guitar due to the forward tuning for the entire bass spectrum. Versus the U2, the bass expression has higher emphasis on the midbass. The U2 is giving me some sibilance on Bob Dylan’s ‘ss’ sounds and a little bit on his ‘t’ sounds. The sibilance is more pronounced on the U4. This track normally has some sibilance, but it is a little elevated with both IEMs on this.

I wanted to get an idea of how the U4 and U2 perform with speedy bass and high levels of complex instrumentation, so I put on Animals As Leaders – Ka$cade (16/44). With the U4, bass tends to go a bit omni-bass, translation: a bass haze that kind of blankets everything. If the shape of the bass curve were the same but at a lower level of emphasis the tuning would be less of a problem because the bass wouldn’t be forward of the mids and blanketing the sound. With regard to all the other instruments, they are generally well represented with the U4 keeping up well on speed and resolution. When only one kind of bass is in the stage, these also do well, but when there is both bass guitar and synth bass, it gets hazy. The U2 has a cleaner bass presentation due to the lower quantity of midbass in the overall presentation. The U2 sounds better on this track even though the stage depth is less. The U2 does fine for speed also.

The U4 has better technical performance when it comes to instrument spacing and stage depth, but it has a tonality problem with vocals that makes it difficult for me to recommend over the U2. If you like an excessive mid-bass, the U4 will work nicely for you. If you like a basshead IEM that is more balanced with less coloured mids and omni-bass, then the U2 is better.

Usability: Form & Function

The Stealth Sonics U4 review has a full break-down of the usability of the universal series, which all share the same shell and versions of the same accessories. I’ve just dropped the ergonomics part in here for reference.

I like that I can change the sound by taking off the faceplate, but I don’t like that this immediately down-grades the appearance. After looking at the construction, I’m pretty sure that damping plays little effect from the faceplate and that it is more likely that opening the screwholes just changes airflow and thus the sound.

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.


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 U2
Price $249
Driver type Two-driver, crossover free hybrid (1DD, 1BA)

DD = low/mid

BA = high

Frequency response 20Hz – 20kHz
Impedance 16Ω @ 1kHz
Sensitivity 103dB @ 1mW
THD ≤1% @ 1kHz
Isolation -26dB
Construction (specific) No crossover

2 bore

Red 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 Stealth Sonics U2 has a fantastic big rumbling bass that is immensely satisfying. It has a forward wall-of-sound character that reduces stage depth and can make complicated passages a bit jumbled, but my lordy is there raw power across the whole frequency spectrum. It’s really enjoyable and definitely scratches an itch.

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