StealthSonics U2 : Square the circle

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 Ross (Jackpot77 on told me about them, and I am very glad he did and included me in the U series review tour. 

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!

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