Headphoneus Supremus
Stealth Sonics U9

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 U2 (link) and U4 (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.


U9 - The clear one
  • Drivers: 1 x DD (Low), 8 BA (2 x Mid, 2 x High, 4 x Super-High)
  • Crossover: 4-way
  • Frequency response: 18Hz - 40kHz
  • Sensitivity: 108dB at 1mW
  • Impedance: 16 Ohms at 1kHz
  • THD: <=1% at 1kHz
  • Price: US$1099


Contrary to the U4, I do feel the U9 are closer to a flat response and I would call it close to neutral, but without becoming dry and loosing their musicality. The U9 are clear, clean and musical with a surprisingly natural tonality to instruments. In that sense there is something of the Rhapsodio Eden to them. The overall tone feels quite bright (in the good sense) and clear, yet instruments still maintain a good body and accurate tonality. Perhaps not quite at the level of the Eden, but those are nearly twice as expensive.

The U9 have a very nice spacious stage that feels immersive and I find works well for classical music. The background is not as black as I would like, but perhaps that has something to do with the Klarity Valve, as the contrast of the notes relative to the background still feels very natural. IEMs with a very black background can have a strong contrast that although it emphasises details and textures, is not actually natural as such. Thanks to the spacious stage and the Klarity Valve, the U9 feel more like listening to headphones even though the stage is not as big. At times the low weight even made me forget I was wearing IEMs. It is a very natural way to experience classical music. The stage does seem to have a mind of its own and I find the imaging of one of my favourite albums, Caro Emerald's Acoustic Sessions, to be off somehow and I have not been able to put my finger on it. Other jazz, like my latest favourite track, Tuba Skinny's Jazz Battle, does feel better, so it could be down to a difference in recording and how that plays with the U9.

Where the U2 and U4 had a prominent bass, the U9 seem to take things a little easier without pushing the bass too far into the background. The bass is tight, precise, has great texture and hits with a healthy dose of impact, but is never overstated. It is exciting and that is very much what the U9 need to avoid becoming dry. The bass sets the rhythm and gets your feet tapping. The kick drum in the Rolling Stone's Hate To See You Go is a great example of this. At first listen with the U9 I did not expect them to be suitable for the Stones and yet the U9 got my bum shaking with ease. The bass is not just impactful, it has great texture and feels pretty fast and articulate for a dynamic driver. It works great with fast drums like in the Foo Fighters' Cheer Up, Boys (Your Make Up Is Running), which makes the U9 surprisingly musical.

I did try the U9 briefly without the faceplate and was quite surprised by the change, which instead of extending the bass made the U9 sound thinner instead. So while I previously questioned whether perhaps the Stealth damping technology did not work optimally, it does appear to do just that for the U9. The bass of the U9 is a very good quality bass that is capable of digging deep and hitting hard when it needs to, but avoids becoming to dominant.

The mids of the U9 are excellent and the reason I like the U9 for classical music. Instruments sound accurate and natural without becoming very full sounding. Now, I do like full sounding instruments, but it is a bit of a disadvantage with orchestral music and the U9 do very well in separating the different instruments. Layering is pretty good and the way the U9 reproduce the emotions of different classical pieces is even very good. While casually listening as I was doing something else, I often found myself getting increasingly distracted by the music, which was pulling me in and enticing me to close my eyes and drift away.

Vocals are perhaps not quite the U9's main strength. The balance between male and female vocals is not bad, although there is a slight preference for female over male. Moreover, vocals lack some definition. Like I indicated with the U2, I am quite spoiled here and so I have set the bar very high, which I think is fairer in the case of the U9 as these are Stealth Sonics most expensive offering. Again I suspect a slight upper-mid or lower treble lift I am noticing here. Nothing too bad mind and simply a consequence of a tuning choice, but if you love vocals as much as I do, I would suggest looking at alternatives. Nonetheless, vocals as such are nice and clear, which can be said for the mids in general as well. Very nice and clear to ensure nothing can escape your attention.

Where the U2 might have been a bit bright in the treble and the U4 a little attenuated, I feel the treble of the U9 is just about right. It is sparkly and yet maintains smoothness. So when I listen to Tchaikovsky's Nutcracker with the U9 there is a nice sparkle, but not too much, and a slight hint of sweetness to keep things smooth, but not too much. A female soprano voice also comes through very clear and clean with the natural brightness that was missing with the U4.

Cymbals sit well within the image and add sparkle op top, but remain in the background. Perhaps some people like a little more treble prominence, but I think this way the signature avoids becoming fatiguing despite the overall focus on clarity and detail. I have had the Rhapsodio Galaxy V2 over for a while, which also had a focus on clarity and details, and those were not for the faint of heart (and a dream for treble heads). I could listen to them, but would get fatigued after a short while. Not so with the U9. The U9 balance everything out very well and unless the bass would get a boost as well, I suspect any additional treble lift would ruin this signature.


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.
Thanks for the review! :) So, based on its lightweight (nice considering they look slightly bulky), the shells are made from some strengthen plastic?
@iBo0m It does feel like some type of plastic, but it does not feel fragile despite the very low weight for an IEM. It might just be psychological, I haven't put their durability to the test, but knowing how strong Stealth Sonics' CIEMs are I would not be surprised if the material was specifically selected for its low weight and durability.


Headphoneus Supremus
Pros: Balanced / Reference sound signature - suitable for monitoring
Very wide soundstage with pinpoint precise imaging
Dynamic driver implementation is tight and controlled
Very high detail retrieval and separation
Superb top end extension
Coherent tuning, despite the very high level of detail and neutral profile, the U9 is always smooth (no sibilance or harshness)
Great value for money
Cons: Lower mids dip means male vocals lack presence
Big shell, protrude significantly
Isolation is only average, even more so with the faceplate off
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 with Custom Art custom silicon tips, I had a perfect fit. I did try the U9 with the faceplate off but contrary to the U4 and U2 I didn’t notice much difference, so the review is based on the U9 with faceplate on.

I didn’t get the U9 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.

Drivers configuration:
- 1 x Dynamic (low)
- 2 x Balanced Armature (mid)
- 2 x Balanced armature (high)
- 4 x Balanced Armature (super-high)
Crossover: 4-way
Isolation: -26dB
Bore: 3
Frequency response: 18Hz - 40kHz
Sensitivity: 108dB
Impedance: 16 Ohms @ 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.

Among the U series lineup, the U9 is the flagship offering a hybrid 8xBA and 1xDD array of drivers and is priced just under 1100$ in its universal form. This fall right into the category of products I think are in a sweet spot of TOTL performance with a sane price thus I was very eager to see what StealthSonics brought to the table in the bracket and compared to other TOTL I own and have auditioned.


The U9 is advertised as having a signature that « is supported by a deep bass response, beautifully transparent midrange and crystal-clear open highs that are powered by four Super-High drivers. Crisp transients and an immersive soundstage round out a truly superior listening experience. » Does that hold true?

Let’s see!

Fit & Build
The U9, 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 either with universal tips and custom silicon tips from Custom Art. The build is superb and flawless, with a nice textured finish that provides a high level of comfort. Isolation is only average, it might not be the IEM for a commute depending on the volume you’re listening to (I know I listen at average volume myself). Beware that with the faceplate off, this is even more true and the U9 is leaking a bit as well (only noticeable in a very very quiet environment but forget listening in bed while your spouse is sleeping :p).


Overall the U9 signature is balanced and defined around its outstanding clarity and transparency, as well as its resolution. It’s a very neutral IEM that doesn’t add any coloration, it’s neither warm nor bright and tone is spot on. Soundstage is very wide and fairly high and deep, elliptic if you will. There is a lot of air by way of its very good treble extension. Bass is controlled with an emphasis on sub bass, mid bass is not prominent. Mids are tuned for accuracy, lower mids are lean with just enough presence for fullness but the flipside here is the U9 is on the thinner side of the spectrum. Despite being highly detailed the U9 is always smooth thanks to a conservative high mids / lower treble region there was no hint of sibilance or harshness whatsoever.

StealthSonics managed to walk a fine line there and find a good balance and many IEMs including flagships failed at this. Keeping a smooth listening experience with a highly detailed IEM is always a challenge. I found the U9 to be fatigue free despite its very high resolution and detail retrieval abilities. Kudos for staying true to their claim regarding the signature as well, their product page sells it for what it is.

StealthSonics has been very disciplined when tuning their dynamic driver with the U9, it’s very apparent they had a clear goal : providing audiophile bass, in the sense that the bass is controlled and detailed, with good texture and good speed for a DD. It won’t give bass-heads a run for their money but it is very high quality and quantity is not lacking either and the snappy attack provides a good bass line and nice rhythm. Decay is pretty fast as well with good recovery time even if it won’t be as fast as a good BA driver.

Interestingly I found bass heavy style to be highly enjoyable on the U9, namely EDM is portrayed beautifully and tracks that usually tend towards messy over the top mid bass become quite articulate. In a totally different genre, I enjoyed strings and especially the double bass on Jazz albums, texture is really good and tone is spot on. There is something reminiscent of JVC FW10000 bass here and it’s high praise in my book.

The U9 mids do not stand out, but in a good way : the U9 is highly transparent. As I hinted, the mids are tuned for accuracy and are neither forward nor recessed.

Interestingly StealthSonics decided to avoid relying on pushing upper mids for clarity and articulation, lean lower mids and upper treble presence does the job already and this would have made the U9 much brighter. This combination grants the U9 a neutral tone, great articulation and separation while remaining absolutely smooth no matter which tracks is played. That’s what I meant when I said mids don’t stand out, so much so that my first impression was all bass and treble, the lack of initial impressions left by the mids was telling.

Vocal placement is smartly put a little forward and vocals are articulate and smooth. The only thing I found myself wishing for is more fullness from male vocals and a tad more body overall but that’s personal preference. The same goes for female vocals, some artists require a bit more upper mids to convey playing on the edge with their performance (think Freya Ridings) and it’s not really there with the U9.

Treble is probably the first thing you will notice about the U9, because it infuses the whole signature with its upper treble : the ability to provide a very high level of detail, the air in its soundstage and precision in its imaging all comes from there. Extension is top notch, attack and decay are fast, which benefits transient speed.

Paradoxically the U9 is not a treble-head IEM because Stealthsonics decided to play safe with lower treble. The U9 could have been more exciting with more prominent lower treble but it would also have made it more fatiguing and it’s not the case at all as the U9 is fatigue free despite its high detail retrieval. The downside is you could find yourself wishing for a tad more bite, but you can’t have it all.


StealthSonics meant to build the U9 to be a flagship model designed for anyone who requires accuracy and I have to say, it delivers on its promise. The U9 does have a « deep bass response », a « transparent midrange » and « crystal-clear open highs ». When a product page states something that holds true, you know that the product was developed with clear intent and craftfully executed.

It would have been easy to stray from the goal and add a bit more lower mids to make the U9 fuller or more lower treble to make it more exciting but although I confess I would have liked them to go for it by personal preferences the U9 wouldn’t have held its initial promise or stood apart the way it does (because then it would have been closer to IEMs I know). I do believe while they didn’t state it explicitly one of the design goals was to make the U9 fatigue free and it is.

The U9 features a very interesting tuning because it manages to be neutral and engaging which - like tightrope walking (yeah, that’s where the title comes from :p) - is a very hard thing to do. It features the right amount of bass to keep you toe tapping while never compromising on its accuracy goal, the main course is in its bass textures and superb detail and control. It doesn’t feature sparkly lower treble and the excitement that comes along with the added bite. Instead it relies on shimmering upper treble to infuse its signature with refinement and air. Last but not least, the U9 mids have great clarity and transparency completing a very accurate portraying of music whichever genre you choose to listen to. For the U9 is an allrounder that can go from EDM to Classical in a heartbeat with ease. While it will provide a great experience with any genre, you might find yourself lacking some bite on electric guitars on Blues and Rock, and most male singers might not sound as powerful and engaging as some other IEM could portray. You can’t have it all :wink:

If you’re looking for a neutral IEM with top notch detail retrieval, great soundstage and accurate tone that provides a fatigue free experience without breaking the bank then the U9 is definitely an IEM you should try! If you’re looking for more bass kick, sweeter mids and sparkle, then other IEMs of the StealhSonics will be of interest. Stay tuned for the U2 review.


Headphoneus Supremus
Pros: TOTL sound quality. Detail, clarity, soundstage, separation, imaging, layering, all superb.
Cons: I honestly found none; as long as this tuning matches your preferences, there's nothing to complain about.

I would like to begin by thanking @jackpot77 and the team at Stealthsonics for arranging this tour and being gracious enough to include me in it :)

I’m reviewing (in separate reviews) their 4-driver all-BA IEM (the U4), and this IEM, the U9. So in the sound description later, there’s inevitably going to be some comparisons between the two; I feel this will be helpful, since they offer rather different sound signatures, with the U4 featuring a pretty monster custom dual BA to handle the low end and lower mids, with the sound signature informed accordingly.
Also, as a relatively new contender on the scene, there may be some potential customers out there wondering which of the Stealthsonics line-up is going to be right for them. So, as always, I aim to please :)

IEM details from the Stealthsonics website:


The U9 by is a hybridIEM featuring, perhaps unsurprisingly, 9 drivers per side.

And, as if a 9-driver hybrid wasn’t reason enough leap with joy and thank God for the joys of consumerism, they’ve thrown in a 4 way crossover and a frequency response range that should allow you to hear dolphins serenading canines on the beach :)

Here we have a combination of a single Dynamic Driver (DD) with 8 Balanced Armatures (BA) in the following configuration:

1 x Low (DD)

2 x Mid (BA)

2 x High (BA)

4 x Super High (BA).

As the last line may make clear, I was only slightly exaggerating about the dolphins :p

Special Features (from the website):

SonicFlo Acoustics: The Aerospace Advantage

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.

Acoustic Optimisation: From Drivers to Damping

Acoustic refinements include extra-large bores that preserve airflow and sonic fidelity and advanced venting features to manage resonance. All of our universal in-ear monitors offer Stealth Damping technology, featuring a faceplate that can be removed to extend bass response for deep, rich low end that you can feel.

Space-Age Materials: Comfort and Confidence

Components are built inside an ultra-light enclosure housed in proprietary composite material that ensures a snug, comfortable, slip-free fit, even during long-term use.

Klarity Valve, Hearing Protection

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 Shells - Breakage Resistant

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.

Regarding these features:

I tried the U9 with the faceplates removed, and at first there was no noticeable difference. Further listening seemed to reveal a very, very, subtle increase in low end weight and rumble. Either way, the effect is extremely subtle (on a par with that of the bass tuning valve on the Mason v3 IEM that I reviewed previously).

I'm something of an audiophile bass-head, and my philosophy on such things is ‘go hard or go home’. Either design it to make a significant difference, or why bother? As with all things, this will come down to personal preference.
In defence of Stealthsonics, they responded that this feature was designed with studio engineers and professional musicians in mind as a kind of fine tuning feature.

In a way, it’s all something of a relief actually, because the faceplates are way too stylish for me to want to remove them anyway :)

I would agree that the isolation (especially with foam tips) is good on these, and the comfort and venting claims are not exaggerated; I’ve worn them for hours without discomfort or in-ear fatigue.

Pricing at the time of writing was USD $1099 which I would say places it somewhere around the beginning of TOTL range in terms of pricing.

And indeed at the top of the range in terms of Stealthsonics’ own universal IEM lineup, making it the veritable Daddy Mac of the group.


This review tour package of 3 IEMs (The U2, U4 and U9) came with only one official packaging; that of the U4.
I imagine the U9 would be largely similar, so I'm including it here to at least provide an idea of what to expect, along with a photo of the copper cable with mic, plus some accessories that were provided (and will be provided with the U9).
The IEM photos themselves are, of course, of the U9 itself.

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Appearance and build:

As an introductory note, the review tour package for these IEM’s came with only 3.5mm Single-Ended cables. I understand that Stealthsonics are working with another supplier to make more cables available. I believe a balanced cable (perhaps with an adapter for SE use) will be included with these IEM’s in future. I would advise those interested to check on their website to see if any updates are released. Either way, they should be available to buy as an extra sooner or later, and of course you can always buy your own or use those you already have.

What this did mean however, is that I could only use the Amp 1 Mk II on my new DX220, with no opportunity to see how these would sound on my modded AMP8 (4.4 balanced).

Nevertheless, the DX220 and AMP1 Mk II are still an excellent combination and provide a TOTL sound, so I’m confident that I’ve been able to give these IEM’s a thorough workout :)

On the plus side, they provide a choice of two cables; a very attractive copper cable with a microphone and a standard silver-plated copper cable of even higher quality. Both cables feel very well engineered, solid and reliable. They are light, flexible and seem non-microphonic when I’ve worn them whilst out and about in the sprawling metropolis.

The appearance of these IEM’s certainly doesn’t disappoint. They have a gorgeous matt black shell, with a shiny metallic chrome trim.

I greatly appreciated the fact that the 2-pin connector for the right hand wire has a red area, still slightly visible once plugged in to help me with knowing my left from my right :p

On top of that, they have a prominent “L” and “R” printed in white writing on the inside face of the shells (hidden during use of course).

The attention to these small details, which are actually quite important in daily use, are a promising sign from this company.

Correspondingly, the 2 pin cables are very easy to remove and insert, clicking smoothly into place, but absolutely secure once in. Very well engineered, as is everything on this IEM.

The faceplates have a very stylish and individual Formula 1 feel, with a glossy and well-executed carbon fibre finish, with what looks like a tiny industrial speaker cone in the middle.

They really look the business and unlike many IEM’s I’ve used, I don’t feel these need to be handled like delicate porcelain due to their very sturdy build and the aforementioned advanced materials. There’s a video of them on YouTube, being dropped from a balcony or something and emerging without a scratch and still working fine :)

One negative (for me) was that the nozzles seem to have a more shallow insertion than that found on my iBasso IEM’s.

As a result, I initially struggled to get a good seal with these.

With my usual IEM’s, I can actually wear all 3 sizes of Symbio wide bore hybrids (my tips of choice in most cases), each offering a different kind of seal/depth of insertion and consequent effect upon the sound signature.

However, with these new IEM’s, I could only wear Large size tips.

The outer part of the ear canal on my left side is pretty cavernous; I’m faintly embarrassed to admit it’s like the Albert Hall in there (other prestigious concert venues are available).
I tried the included silicon tips, double flange tips and memory foam tips.

Only the memory foam tips gave me a decent seal in that ear.

However, it made the sound a bit veiled, which is a side effect that can happen with foam tips sometimes.

My Symbio tips could get a seal, but it always slipped after a few mins.

Disaster! A two and a three! (a ‘Risk’-based joke from TV show Red Dwarf) :p

However, I then found an old pack of memory foam tips of my own.

These have lime green wide-bore cores and were bought cheaply on eBay.

Now I have great seal and insertion, without any deleterious effects upon the sound signature. And now, the IEM’s start to shine.

Victory! All of your bases are belong to us! :)

The Sound:

As ever, my preferred method of testing is to try out the product(s) in question with a selection of songs from various genres and to let that process draw out the comparative strengths and weaknesses of each product, with a tl;dr summary at the end for those who lack my infinite saintly patience :wink:

I have a few tracks which I’ve only found available on MP3; the rest are FLAC or WAV in 16/44 or 24/192.
For the purposes of this review, I used the iBasso DX220 (with AMP1 Mark II).

Note that I have been reviewing the Stealthsonics U4 as well, so I’m including some comparisons in my notes here, since it represents a rather different sound signature with a powerful and emphasised bass and more warmth and note thickness and weight as a result. So by including these comparisons, I hope it could help people to better figure out whether the sound signature of the U9 is ideal for them or not.

The Sound:

Hanson: Change in my Life (16/44 FLAC).

This is an acapella song, featuring harmonising of the excellent voices of 3 brothers who’ve been singing and performing together for about 30 years.

This is a useful test track for testing midrange and seeing how well the IEM can handle the 3 voices, individually and as a group.

What stood out for me here, compared with listening with the U4 previously was the size of the soundstage, and the top-class separation and imaging.

It was significantly easier to pick out the separate voices as they harmonised.

Timbre seems on point, although I wonder if it might be improved with a tad more low end to lend some weight?

The overall effect though is of a significant jump in quality and performance

Eurythmics – Angel (HDTracks Remastered 24/48 FLAC):

Wow. Wow!!

I’m familiar with this song, but 16/44 FLAC (in the original non-remastered version) is the best I’ve ever heard it until now.

I recently got this HDTracks remaster, and it sounds superb.

A song that I already loved is just taken to new heights.

Fans of Eurythmics songs; I highly recommend you to check these out!

The remarkable mastering of the song makes the soundstage and separation sound big here; combined with the large and holographic soundstage of the U9, it’s like a sonic cathedral of sound :D

The sounds of the acoustic guitar fingerwork/plucking and the percussion are very pure and crystalline. Every detail here is presented like diamonds on black velvet – this song REALLY shines on the U9. So much detail and clarity, superb layering.

The Ataris – So Long, Astoria:

This high tempo pop-rock song features crunching guitars with lots of percussion. Not a bassy mastering, but an otherwise fairly dense ‘wall of sound’ kind of song.

The U9 captures very well the ‘tizz’ of the high percussion.

However, this song doesn’t have a strong bass in the mastering, which actually I feel is somewhat detrimental to the track, given that it’s a loud, driving rock song.

As such, on tracks like these I personally prefer an IEM with an extra-powerful low end that can provide that power and drive that I crave.

The U4 delivered, but at the cost of sounding slightly congested and bottom-heavy. No such issues with the U9; spades of separation, detail and fizz, but at the cost of having a thumping low end that really let the song rock hard.

Farhan Saeed & Shreya Ghoshal – Thodi Der (from the Bollywood film ‘Half Girlfriend’ OST. 16/44 FLAC):

This song is gorgeous. The first time I heard it, I was entranced. And I still am each time.

It’s very pure, clear, beautiful. It takes a good combo of equipment to display this to perfection without letting the highs become piercing or strident in occasional places.

It starts with a high-pitched female vocal, then a deeper, but sweet male vocal, and at times harmonises both beautifully together. Very moving.

I hear very good timbre on both; the U9 deals very well with both male and female vocals here. The background music has plenty going on with a variety of different stringed instruments, and these are all presented very well; a lot of detail, accurate decay, and the holographic sound signature allowing them to shine.

Alison Lau – Handel’s ‘Il Trionfo del Tempo e del Disenganno’ (HDTracks 26/96 FLAC):

A staggeringly tranquil and beautiful piece of opera.

I am embarrassingly new to the genre and don’t know much, but I’ve started picking up bits and pieces. If anyone knows of any similar opera tracks that I might like, please do PM me or respond below :)

This track stunned me when I first heard it. It was literally a transcendent moment. Aided by being a high quality HDTracks version, I was excited to see how the U9 might perform with this track.

I was not disappointed. The singer, Alison Lau from Hong Kong, has a terrific and versatile voice, able to sing low and soar high from line to line.

What I got from hearing this track on the U9 was how well it brings a sense of space and realism to the presentation.

The baroque instrumentation doesn’t sound quite so rich and resonant as it did with the U4. But the U9 still captures the timbre and rapid changes in the vocals excellently, handling the transients well where the song featured pauses and moments of silence as the instruments trailed off.

Never becoming sibilant or shrill in the high notes, although those 4 super high BA’s are really taking it to the limit beyond which I (with my treble sensitivity) might start to find it uncomfortable. As a result, I couldn’t turn up the volume as high as I could on the U4. I doubt this would affect most people though, and I would stress that this was not an issue; it might have been were the tuning any more sharp. However, I think that Stealthsonics have walked the fine line here admirably.

Sidney York – ‘Dick & Jane’ (from Korean drama ‘Age of Youth’ OST, 320k MP3):

Ha, this is a bizarre song. Foot stomping percussion (literally on this recording), madcap vocals, a ukulele, whistling, and a ridiculous bassline that oscillates wildly in a way that only The Smiths could comprehend.

It should be awful, but somehow holds it together for a goofy fun ride.

The combination of all these things make for a tricky customer for any IEM, but the U9 does very well here, bringing out the detail of all the various things going on and never becoming overwhelmed or losing its sense of balance and poise, which would be very easy to do here. Also, here the reduced bass (compared with the U4) and the larger soundstage and separation really help with this congested track, making is sound much more open and accessible.

Shawn Mullins – The Gulf of Mexico (16/44 FLAC):

So, this is one of my go-to tracks for critical listening.

This is a song that sounds terrific where there is huge soundstage and separation or shimmering highs. Ideally both; it really brings out the guitar strumming When there’s both, it’s stunningly good. My iBasso IT03 and IT04 IEM’s do this superbly.

Here, the song sounds terrific. The soundstage and separation on the U9 are just what is needed, and those 4 super-high BA’s really bring the shimmer and sparkle required, with a big dollop of extension. Timbre again is excellent.

Garrett Kato – Love is an Advert (16/44 FLAC):

Discovered this guy via the soundtrack of Australian/NZ tv show ‘800 Words’.

Really love this song; just good modern singer-songwriter stuff with a solid backing band. Very uplifting song. It sounds especially good with the U9.

Its openness and holography are a perfect complement to bring out the best in the song. The drums are presented with authority and precision. The timbre on every guitar strum and pick is top-drawer.

This is a match made in heaven :)

Miles Davis – Blue in Green (HDTracks 24/192 FLAC):

A jazz classic, blissful and folorn trumpet circling like a solitary sea bird over tranquil waters of brushed percussion.

It’s a rather different presentation than I found on the U4; dare I say a tad more ‘audiophile’? Again, the details are just sharper and clearer, but without sacrificing smoothness. The timbre is more accurate, albeit lacking a tiny bit of the warmth and body that the U4’s low-end brought.

Honestly, this sounds superb with the U9. Gorgeously mellow, complex yet coherent.

Dire Straits – Sultans Of Swing (DSD 64):

One of my go-to test tracks for testing (and just listening), and I know several other reviewers on here share this point of view!

A terrific song, with lots of things to get analytical over; percussion, timbre, guitar fretboard wizardry etc.

So right from the beginning, we have a kind of strummy/plucked guitar riff, with another guitar solo-ing over the top.

The U9 presents this with more shimmer but less weight and body than the U4. The shimmer adds a nice musical feel to the whole presentation, although I’d say the warmth and weight of the U4 made for a rather different (but equally good) feel.

I think the combination of the U9’s expansive soundstage and comparatively more analytical sound has a tendency to draw me in to focus on the details that it captures so well, whereas the U4 lets the details shine but without causing me to lose my overall focus on the music. Both are good; it really just depends on your preferences.

Anberlin – The Art Of War:

This is an immense track. There’s so much going on in it and so many things to zone in on when listening. There’s powerful percussion and bass, a driving rhythm, synths and sound effects and over all this, great vocals, searing lyrics and simply majestic rock!

In terms of analysis of detail retrieval, on this track, around 9 seconds in, there’s a sudden sense of space opening up in the upper-central zone of the soundstage, along with a faint, almost imperceptible hum.

I don’t know much about music production, but I’m guessing this is the ‘channel’ being switched on that the bass guitar is linked up to (as indeed the bass comes in at the same spatial location a second or two later).

This small detail of the channel opening up for the bass at 9 seconds in is captured very noticeably, even more so that with the U4.

This is a modern style of rock, dynamic and relatively rich in the mastering.

The U9 handles it very differently than the U4 or other IEMs with a powerful low end would.

The song starts with an electronic pulsing beat, which the U9 presents with a good level of tactility.

Then the bassline comes in.

Here, the more ‘normally’ tuned low end leaves this song without the astonishing power and impact it can have with the most suitable IEM’s but it is still an excellent presentation with its own unique pleasures. Every detail large and small is captured with immense precision and sharpness. That pulsing electronic beat has a real edge and dynamism here with the U9, and the U9’s big and holographic soundstage opens the track right up so that even in the most dense parts of the song, it never becomes congested. Layering and imaging are top class.

The Cranberries – Twenty One:

This was a delight. The song features shimmering strummed guitars, accompanied by picked electric guitar with some effect whose name I forget :p

Every guitar line stood out and was rendered superbly.

The detail and presentation on this IEM are absolutely top of the line.

It reminds me of the EE Zeus, with a little bit more weight in the low end.

There’s a lot of airiness and treble extension on display here. Superb.

Summary of the sound signature:

It's probably just my personal bias, but I always feel faintly cheated and offended when I encounter a dynamic driver IEM that doesn't have walloping bass, given the ability of the DD technology to deliver a stunning, visceral, tactile low end.

It seems like an affront against nature or something; like having a Lamborghini with a capped speed limit or engine noise dampers :p

The low end took a bit of getting used to after 5 days of bass-cannon tomfoolery with the U4.

Once I’d gotten used to the U9’s rather different sound signature, I could see (or rather, hear!) that there is in fact a decent amount of weight and depth across the bass spectrum, allied with excellent control and focus.

Certainly, where the bass is there in the mastering of the recording, it presents it with a good level of power and authority.

It’s just not going to add bass where there isn’t any, but I imagine for the majority of audiophiles, that would in any case be an undesirable quality in most cases.

The mids are delightfully balanced, neither too forward nor recessed. Timbre is excellent, especially as we move up the frequencies.

The treble is a thing of wonder. It has that pure crystalline quality, lots of air and extension, and a rewarding shimmer and sparkle that lights up songs.

Micro-detail is world-class (and superbly presented with stunning clarity); imaging, layering, separation and soundstage are all outstanding.


I found no significant weaknesses here. As with most TOTL equipment, it’s all about one’s own sonic preferences.

As something of a basshead, I could wish for a more thunderous low end, but in doing so, I’d inevitably be trading off some of the things I love most about the U9. Having had time to get used to it, I think the bass is a tad north of neutral and has a satisfying weight and power on all but the most demanding tracks.

I’d say there is more of a focus here on clarity, detail and precision than on richness and thickness. So again, depending on your preferences, this may be Top of the Pops or Room 101 (British TV references, if you are confused).

Overall, I can only praise the U9. It’s been an absolute treat to spend time with it. At its price point I would say it offers comparatively very good value.

I wouldn’t see it being ashamed or inferior in comparison with other TOTL IEMs costing double the price.


The U9 offers TOTL performance at a very competitive price.

It combines terrific detail, clarity, extension and outstanding technical performance with a hugely enjoyable neutral-musical signature, all wrapped in a large and holographic soundstage.

If this sounds like what you’re looking for, then the U9 must be a serious contender for your currency :)

I hope there’s a U1 out there somewhere, just so they can say that the U9 is at least nine times better.

And on that note..

I think pretty much all those IEM's you mentioned have a tuning that features a more impactful and hard-hitting bass than the U9. If BASS (in an otherwise audiophile tuning) is what you're looking for, I'd suggest that the U9 may not be for you. U9 is tuned more towards sub-bass, so there is some impact and depth there, but it's not tuned as powerfully as the others you mention.
Conversely, when it comes to the rest of the sound signature, I think U9 can hold its own against any of them. At this TOTL level, it would be foolhardy to say "better than", because it mainly just comes down to sound signature preference. Of the Empire Ears line-up, I'd say Zeus and the new Valkyrie (mids & treble, rather than bass) are the IEMs most similar to the U9. Expansive soundstage and vast, extended, open treble.