Headphoneus Supremus
Stealth Sonics U2

I would like to thank Stealth Sonics for providing the demo units for this tour and Jackpot77 of Audio Primate for organising the UK tour and letting me participate even though I escaped back to the continent last year. No incentive was given for a favourable review. This was originally posted as a single review, but adapted to fit the proper Head Gear sections, therefore certain sections will be the same as for the U4 (link) and U9 (link) reviews.


Stealth Sonics is a company based in Singapore and while a relatively new player in the IEM market, has had a decade or so experience in audiology/music technology in Southeast-Asia. I first came across them around the NAMM show in 2018 where Stealth Sonics launched their new IEMs. I enjoy occasionally browsing around to see what is new and what might peak my interest and Stealth Sonics certainly did the latter when I saw their universal IEMs. A bold design that made a statement: StealthSonics has Style. That is style, but then with a capital 'S', quite possibly it even needs to be italicised as Style. Whether or not it is your cup of tea is another matter altogether, but there is no denying that Stealth Sonics sets itself apart from the crowd. It is an extravert styling with bold colours and design elements, some of which are also functional (more on that later). For me, I am about as extraverted as a particularly agoraphobic hermit crab, so not quite my cup of tea although I do appreciate it when companies have their own unique Style.

It is not just the Style that sets Stealth Sonics apart. They have some really innovative ideas too. For one, their custom shells are coated with a proprietary lacquer that makes them extra hard. So hard that they can survive quite a large drop on a hard floor and that is not something I have seen very often. The universal shells too have some nifty design elements of which the Stealth Damping technology is the most noticeable because it looks like someone stuck a turbine engine to the faceplate. It is meant to help create a tight and clean bass response. The faceplates can however be removed to extend the bass, or replaced to make your universal Stealth Sonics IEMs look even more eye-catching.

The tour package contained the full retail packaging of the U4 to give an example of what you can expect when buying any of the three universal IEMs. So while I can only show the U4's unboxing, the experience should be the same for the U2 and U9 as well.






The universal Stealth Sonics come in a relatively big box covered by a sleeve with on the outside a picture of the IEMs and specifications. A bit more information can be found on the inside of the sleeve as well. The box itself has a carbon-like look and feel to it and opens up to display the universal IEMs in all their glory. Alongside can be found replacement faceplates. In the case of the U4 these are glossy blue, and I believe the U2's are red and the U9's black. Standard on all three are carbon-look faceplates. Below the IEMs is a generous size case, which contains a second cable with a mic, a bag with various ear tips (foam, silicone and double flange), adapters, an allen key for the faceplates, a microfiber cloth and a pouch. Overall a very healthy selection of accessories.

I believe that the regular cable is an SPC, although with all three IEMs it has a different colour: black for the U2, blue for the U4 and silver for the U9. The mic cable appears to be a pure copper one with clear insulation.


Build quality and fit
As I indicated earlier, the Stealth Sonics CIEMs have a uniquely strong build quality thanks to a special lacquer that strengthens the shells. The universal IEMs do not seem to be far off in that respect with a very durable feel to them, but it is a little deceptive because they are extremely lightweight. This low weight is purposely done to improve wearing comfort because Stealth Sonics aim at achieving long listening sessions (6-8 hours) without inducing signs of fatigue. Comfort is essential here and I do find that the low weight helps the IEMs to disappear while I wear them. The shell material below the faceplate also feels very soft and is very comfortable when wearing. The fit though was a little tricky for me to get right and I ended up using Final tips one size above my normal to get the best seal and most secure fit. I am not sure if I got the optimal fit, but time constraints meant that I could not spend too much time tip rolling. The reason it is a little tricky is because the stems are quite thick and short, so the fit becomes relatively shallow.




The included cable is quite a good one and comfortable to use, although the long ear guide did occasionally push and pull a bit more than I am used to because the cable did not settle around my ear. The cable's ear guides made the cable "float" (as it were) just above my ears because the shape of the bend is held very well, rather than something suppler that settles on the ear. Other than that it is certainly a quality stock cable that I think most people will be very happy with.

As mentioned, Stealth Sonics aim to achieve long, fatigue free listening sessions for their (C)IEMs and have fitted them with what they call a 'Klarity Valve'. This is a type of pressure relieve valve that helps minimise pressure build up during use. The vent of the valve can be seen on the inside of the shell, just alongside the 'L' and 'R' indicators of the monitors. I have had issues with pressure build up in the past and I find that the Stealth Sonics IEMs are about as comfortable as using IEMs with APEX/ADEL modules. I did not get any uncomfortable pressure build up and was quite happy to have the IEMs in my ears for longer periods of time.

All listening was done with the Cowon Plenue 2 from the SE out.


U2 - The fun one
  • Drivers: 1 x DD (Low/Mid), 1 x BA (High)
  • Crossover: None
  • Frequency response: 20Hz - 20kHz
  • Sensitivity: 103dB at 1mW
  • Impedance: 16 Ohms at 1kHz
  • THD: <=1% at 1kHz
  • Price: US$249


The U2 have to my ears a mildly U-shaped signature with a solid bass that is balanced by a sparkly treble to add a bit of fun and musicality. The stage of the U2 is not the biggest, providing an intimate setting that is complimented by a somewhat holographic feel. It makes the U2 immersive and fun to listen to. While I would say the overall feel of the signature leans towards warmth, the U2 manage enough air to provide good separation and I did not notice much in the way of congestion, even with layered choral music. The background though is not the blackest I have come across. Perhaps I am spoiled, but the image lacks some crispness and definition. Not much of a problem though because the signature is musical enough that it will compel you to tap your feet nonetheless, making the U2 really good fun to listen to.

The bass of the U2 surprised me a little bit because it dug deep, was impactful and tight and yet I missed something I could not quite put my finger on. I suspected that this might have to do with the Stealth damping that Stealth Sonics incorporate in the faceplate and so I did a quick test without the faceplates, which immediately felt like the bass extended a bit better and resonated more naturally. I did not have enough time to do a thorough comparison, but it might be interesting to try out for anyone who enjoys an even deeper, lusher bass. I thought it worked quite well for the Rolling Stones, putting a bit more emphasis on the kick drum and bass guitar, but that might not always be what I aim for. With the faceplate on, the bass becomes tighter and more impactful, something that worked better for EDM. Although I will admit that my inner bass head always enjoys a deeper, lusher bass, even when it is strictly speaking not ideal. Of course you can have both in this case due to the U2's option to remove the faceplate or leave it on. It is however fiddly to do and I would prefer a simple switch (if it were at all possible with this particular system).

The mids are pretty decent. They are not the most natural sounding, but I will immediately state that as a mid-lover or 'mid-o-phile', I am very picky when it comes to my mids. The U2 sound far from artificial or thin, but lack some fullness and definition. Perhaps the U2 have a little too much emphasis on the upper-mids or lower treble, which is particularly noticeable through a lack of vocal density. In choral pieces such as Bach's Magnificat I find that male vocals lack some of the power they need to balance out the female vocals, but female vocals too lack some definition. Agnes Obel just does not sound like I expected based on the intimate setting the U2 produce. I might need to emphasise here that I do not have other IEMs in this price range for a comparison, so perhaps my bar is set to high and I am just a spoiled little brat because of the much more expensive, mid-centric IEMs I am used to.

Other than that the mids are nice and clear and even with classical music, which does not play to the U2's strength, I find that instrument tonality comes through very nicely. Move to proper fun music such as rock and the U2 perform really well.

The treble is a hint towards bright in order to provide the sparkle that compliments the fullness of the bass really well. It adds a little bite to guitar strings and indeed electric guitars sound pretty good. The sparkle and bite are not overly prominent and I don't think will give many issues to even the more treble sensitive souls among us. (Guilty as charged.) I always like to risk my life for my readers by testing for sibilance with a variety of soprano voices that are high enough to shatter my glasses even when I am using IEMs, and it was quite smooth with the likes of Elin Manahan Thomas.

The treble I feel is just about right to provide balance, a little bit of air and some clarity to the signature and it works really quite well.


Stealth Sonics have come up with a very nice trio of universal IEMs. The U2 have a signature that is engaging and fun, and I am disappointed I did not have the Final E5000 around anymore because it I suspect the U2 would give those a fair bit of competition. The U4 feel to me like great quality stage monitors that offer the complete package, a smooth and fatigue free signature combined with a very light weight and all the advantages of the Klarity Valve to make them as comfortable as it gets for long, very long listening sessions. The U9 offer clarity and detail at a very high level, while maintaining smoothness and accuracy in the reproduction of instruments. Where the U4 feel like stage monitors, the U9 might lean more towards studio monitors, although without foregoing musicality.

Stealth Sonics offer all their IEMs with a healthy selection of accessories and pack their IEMs with unique technology and styling to set them apart from the masses. At their respective price points the U2, U4 and U9 are interesting propositions, especially for those who prioritise practical aspects such as durability, comfort and a fatigue free listening experience. I feel Stealth Sonics have done an excellent job to create such a complete package and look forward to what they will do in the future.


Headphoneus Supremus
Pros: Bass well extended with good textures and control (with faceplate on)
Great soundstage with very good width, surprising height and good depth
Superb transparent and smooth mids, punching well above its price bracket
Lively treble with good sparkle and energy, as well as decent upper treble extension providing air and resolution
Cons: Depending on source and cable bass can be too dominant in the signature (underpowered source typically), masking superb mids and overpowering the signature
Faceplate off is not usable, bass looses too much control
I’ll receive a free StealthSonics review unit among the U series lineup in exchange of my honest opinion on two of the U series lineup.

Listening notes
This review is based on over 20 hours listening through several sources : DX220 with AMP1 mK2 and AMP9 mostly with PW Audio 1960 cable 4 wires unbalanced, AAW Capri lightning cable and iFi iDSD Micro Black Label.I listened mostly to AMP9 with the U2, and with faceplate on.

I didn’t get the U2 packaging for the review tour but based on the U4 packaging I can say the packaging is premium and the carry case is of great quality.

Driver configuration
1 x DD (Low/Mid)
1 x BA (High)
Crossover : none
Bore : 2
Isolation : -26dB
Frequency response : 20Hz-20kHz
Sensitivity : 103dB at 1mw
Impedance : 16ohm at 1KHz
THD: <1% @ 1kHz

The folks at StealhSonics are a « group of audiologist, engineers and musicians that have been serving the audiology and audio needs of musicians, audio professionals, audiophiles and patients in SouthEast-Asia for almost 10 years ». I confess I hadn’t heard of them before @Jackpot77 (Ross) told me about them, and I am very glad he did and included me in the U series review tour. This is the second review after the U9 (https://www.head-fi.org/showcase/review/22336/).

Among the U series lineup, the U2 is the entry point offering with a hybrid one DD and one BA configuration with no crossover, just under 250$.

The U2 is advertised as having a signature that « delivers warm, musical detail with a signature supported by a full bodied bass and smooth midrange and high for a well textured presentation » Does that hold true?

Let’s see!


Fit & Build

The U2, as all of the U series, has quite a big shell in its universal form. This could be an issue for smaller ears and even with bigger ears the IEMs protrude significantly like the Solaris although to a lesser extent. This contrast with a shorter than average nozzle length, but that was not an issue for me. The build is superb and flawless, with a nice textured finish that provides a high level of comfort. Like the U9, isolation is only average with faceplate on and even less effective with faceplate off. The U2 might not be your better option for commuting depending on how loud you listen to music.

The U2 feature a well thought out warm and smooth signature, that doesn’t sacrifice detail retrieval and has good sparkle up top. Soundstage is not as wide as the U9 but it’s more holographic as it’s both taller and a bit deeper. Compared to IEMs in the same range, it’s very good and even more so considering the signature. I’d go as far as saying that because of its bold bass, the U2 has paradoxically grander soundstage than the U9 with marginally less width but great height and a bit more depth.

Indeed the bass presence dominates the signature, it’s a full bodied bass with significant amount of mid bass but also a very good sub bass extension. Mids are full and lush, but not overly warm which was a good decision as the U2 would have been bloated otherwise. Upper mids have enough presence to make for an articulate listen despite the prominent bass in the signature. Last but not least, treble has also been tuned to provide some needed sparkle and air as well as detail and separation. You won’t find the superb upper treble air of the U9 obviously, but the detail retrieval is notably very good given the bass dominant signature.

As far as source matching go, the U2 will respond favorably to neutral, open sounding sources and the best match for me was iBasso AMP9 on the DX220. AMP1 mk2 was also quite good but less refined and with less air. AAW Capri lightning cable and iDSD Micro BL did very well but not as good as NuTubes powered AMP9 which brought the textures to another level entirely as well as great refinement up top.

The dynamic driver is clearly doing its job : the U2 bass provides real bass-head kick, bass-head rejoice! This being said, StealthSonics tuned the U2 not only for bass quality but also quality : textures are very rich and it’s a nuanced bass with detail. Given bass quantity and the lack of crossover bleed into the mids could have been a concern but luckily StealhSonics did things well here.

On top of this, the use of the Stealth Damping Technology does work well with the U2, only not necessarily in the way you’d think. StealthSonics advertises an extended bass response with the faceplate off and that’s true. It also does help the soundstage expansion and the U2 « breathes » better but to me the removable faceplate mode is simply not usable when it’s off because you loose too much bass control. On bass heavy tracks (Alice Jemima « Licorice » is an example) the bass just goes overboard and becomes messy. After a couple hours with faceplate off, I switched to faceplate on and never went back.

Bass control is greatly improved with faceplate on and to me that’s how the U2 is meant to be. That’s even more true with warmer sources and/or sources featuring a bass boost where the U2 can gain too much bass quantity masking its other great qualities and in particular its great mids.

Whichever mode is used the bass remains smooth as it’s not the Vega type hard hitting bass either : attack is on the softer side and decay is on the longer side. It’s therefore not a fast bass but rather a smooth, rich, textured bass.

There is consistency in the philosophy of the mids with StealthSonics, mids are fairly neutral and transparent. The U2 is neither forward nor recessed and luckily StealthSonics didn’t boost the lower mids - that would probably have been too much - combined with the fat mid bass that dominate the signature.

The mids are articulate thanks to a smart upper mids tuning. Vocals have good presence which was important given the dominance of the bass line. Timbre is really good as well, instruments sound natural this becomes really apparent on bass light tracks (Folk with acoustic guitar comes to mind, think Amber Rubarth « Sessions from the 17th ward » and a cappella think « The Persuasions » albums). It takes this kind of bass light tracks to really figure out the mids on the U2, and I have a list of those tracks for bass heavy IEMs that tend to mask the potential of the mids with their bass presence.

Let me tell you : the U2 really has superb mids, this is not as apparent with heavier bass tracks but it’s there alright and it’s superb and up there with top tier offerings. The « silky midrange » advertised by StealthSonics is spot on, and then some as you can add natural and accurate as outstanding qualities. Impressive for the price range.

The picture of the U2 signature wouldn’t be complete without its treble, and StealthSonics has done a nice job here. Contrary to the U9, the focus is not on the upper treble although they are quite better than I expected on the U2 and provide welcome air and help the U2’s resolution and soundstage.

This being said focus is rather on lower treble and there is a fair amount of sparkle and energy, while remaining smooth. This energy gives the U2 some welcome bite to electric guitars or snares, as well as provide a nice counterweight to the ever dominant presence of the U2’s bass. The U2 is not a dark IEM but could have been if not for its treble presence and performance.


The U2 is quite an interesting entry into the StealthSonics lineup, it keeps what appears to be the brand’s fundamentals which is a knack for smooth signature, transparent mids and high ability for detail retrieval, while adding a strong and qualitative bass presence. A square of the circle, if you will...

If you’re looking for a smooth and fun sounding IEM with strong bass, beautiful mids and lively treble that does not compromise on fundamentals and is showing top performance in their price bracket, the U2 is definitely worth an audition! « With or without plates » (yeah I had to do a U2 reference somewhere ;p)? I’d advise with rather than without, as the damping technology is working, the faceplate does bring a lot more control to the bass and it’s much needed.

There are few gems in the sub 500$ range and the U2 is one of them in my book, there is a high chance that it will land in my collection along with my FIBAE Black. But I wonder how the custom U2 aka the C2 differs from the U2 in this regard as there are some proprietary technology added in their custom IEMs and it might influence greatly (and potentially positively) the signature and I like the U2 so much already I could very well go ahead and purchase the C2. If I do, I’ll sure get a comparative review included here :)

Have to disagree on the mids presentation. Almost any non-warm driver performs well in the mids if you eliminate the big bass elements. These don't do well when you have lots of stuff going on. The instruments blend and get muddled--it could use a crossover (it doesn't have one). For me the key to mids presentation is that it stays clean under duress, these don't. Tonally these are good throughout, though. I've also got different opinions on the faceplate.
Hmmm, I don't hear the U2 has having mids muddied by the bass (no bass bleed) at least no with my sources, the mids are still there and still very good IMHO it just doesn't do them justice to have prominent bass shadowing them by their pure quantity in the mix is all I am saying. Again, note that I am listening with custom silicon tips which definitely has an impact on bass presence and overall performance.
I'll check back with universal tips are this probably affects greatly the end result as usual with universal tips. Last but not least, I am unsure at how long the DD has been burnt in total since they shipped. Sometimes requires quite a bit of time before settling in or at least this is my experience with DD.