64 Audio Duo

I don't write reviews for IEMs I don't like, and I've tried plenty. But I am unapologetically in love with Duo. Purely for sound quality, it's now my favorite all-rounder IEM ever. I'm listening to the Beatles right now, and it's pure magic.

Having owned what is perhaps the most legendary 64 Audio IEM, the U12t, the Duo is quite a surprise for two reasons. First, the U12t is a benchmark IEM, but I'm not alone in feeling Duo's musical tuning is somehow even more enjoyable.

Second, it's surprising that Duo is no longer in production. My theory is: people may look at the other 64 Audio models and wrongly assume that with only two drivers (DD, BA) the Duo doesn't perform as well. Moreover, 64 Audio may have been making room in their lineup for their new planar IEM, Solo. But I can assure you, Duo is the epitome of a hidden gem.



Duo offers something unique. It has an innovative semi-open back design, which makes a genuine sonic difference. The soundstage is insane... grander than any IEM I've heard. It reaches outside your head, naturally immersing you in music like a headphone.

While the soundstage is its most noticeable attribute, Duo may be even better at some other things. Part of its genius is how its Tia drivers eliminate sibilance. Like completely. If you look up its graph, there is a dip right at 4khz, which happens to be the sensitive area for me and for a lot of people. But the treble is airy and crisp, and definitely has sparkle, while remaining smooth even at high volume.

Next, it is remarkable that a semi-open back earphone has this kind of bass performance. Evidently the depth and power of the low end is unaffected by the open back. Duo could be called an audiophile's basshead IEM because of its accompanying characteristics.



For instance, Duo's balance is every bit as remarkable as the U12t, along with a richer midrange. Mids are important to me, as I'm sure they are to you. For the last two years, I have enjoyed several Penon IEMs. Penon has a well-deserved reputation for beautiful mids. Duo has euphonic mids as well.

Partly because Duo has a dynamic driver handling the mids, and partly because the soundstage is widened by the open back design, the mids are breathtaking. In contrast, the U12t's stadium-like soundstage was accomplished by having twelve balanced armatures in each side, but also by pulling back the mids a tad. This tuning choice succeeded in putting the focus more on the surrounding soundstage. But the mids suffered a bit.

Duo is U-shaped, but its mids are different because of the dynamic driver timber. Vocals have a bit more meat on their bones and sound natural, giving Duo more of a W shape on a lot of tracks. Then the open-back naturally places vocals within what sounds like an amphitheater. Music is therefore rendered in a more emotive way than the super technical U12t. Duo puts musicality first.



Don't get me wrong, U12t is also musical, and tuned to near perfection. But for me, Duo's sound is literally perfect. I have never said that before. We often say in this hobby that no IEM sounds perfect. Well, to me, Duo actually does.

Comparisons

I've owned a number of comparable high end IEMs, including:

64 Audio U12t $2000
AE Mentawai $1800
Penon Voltage $1199
Symp.Titan $999
Jomo Nautilus $800
FlipEars Aether $699
Timsok ts-316 $600
Pen.10th Anniv. $500

Duo is my favorite of all. The only IEMs I've heard that meet and at times exceed the performance of Duo are the Penon Impact at $2500 and Mentawai at $1800. I also gave them well-deserved 5 star Headfi reviews. I admittedly haven't heard any 3-5k IEMs, although Impact and Mentawai are said to compete with the likes of Traillii and other giants.

Impact is a slightly superior technical IEM than Duo for resolution and separation, and the soundstage has more depth, but less width. But overall, I prefer Duo. I've never immersed in music with an IEM the way I do with Duo. On the other hand, I've also never heard such flawless resolution as with Impact. It is an absolute technical beast. But Duo sounds more natural to me, and the resolution is close. It suits my sonic preferences more ideally.



Here's a chart comparing my opinion of Impact and Duo, 10 being the best I have heard.

Impact: $2499

1. Tonality: 10
2. Musicality: 9.5
3. Treble: 9.5
4. Mids/vocals: 9.5
5. Upper mids: 9.5
6. Bass: 9.5
7. Soundstage: 9.5
8. Resolution: 10
9. Build/Aesthetics: 9
10. Price/perf: 9

Average 1-9: 9.55
Av. with price: 9.5

Duo: currently $999 new

1. Tonality: 9.5
2. Musicality: 10
3. Treble: 9.5
4. Mids/vocals: 9.5
5. Upper mids: 10
6. Bass: 10
7. Soundstage: 10
8. Resolution: 9.5
9. Build/Aesthetics: 9
10. Price/perf: 10

Average 1-9: 9.66
Av. with price: 9.7

All of the aforementioned IEMs had something I would change, except Duo and Impact. For example, with Titan's true basshead tuning, the mids and treble quality suffered. And with the Ts-316, Voltage, and Nautilus, they are excellent IEMs but have varying levels of harsh upper mids. I would like to hear the lovely Mentawai again now that I have the Cayin N7 for my source. It was spectacular with my Mojo 2, but could be fatiguing.

But Duo has no sonic flaws to my ears. I often use EQ with IEMs, but I feel no need with these. Everything is wonderfully balanced and never harsh. If you enjoy high volume listening but have upper mids sensitivity like me, try Duo. Some might say the treble is rolled off, but I hear plenty of sparkle. I think the smoothness is owing to the brilliant Tia driver. And because of its Wave Sync technology, Duo sounds every bit as cohesive as my single DD Ts-316.

I gave Duo a 10/10 for bass because although I've heard more visceral bass with Titan, it felt disproportionate. Duo is a bassy IEM, but it is extremely well-balanced. Its DD exudes a pleasant warmth through all genres, with crazy slam when the track calls for it. Tri Clarion Wide Bore and Penon LS tips remove any occasional bloat that some have mentioned.

And back to the U12t, Duo achieves nearly its level of pristine detail, plus you get emotive mids, an even wider soundstage, and deep DD bass. The resolution is surprisingly good, plus you gain a dose of Duo's secret sauce. Familiar tracks are infused with something magical.



Song Impressions

Wish You Were Here, Comfortably Numb Live at Pompeii by Pink Floyd: My 2 favorite live songs, but now I'm in the stadium, 4th row. With Duo, live recordings sound like my open-back headphones. Bass notes feel like they are coming from a PA system. The accoustic/ electric guitar and vocals are textured, with no harshness. 🏟

Starboy by The Weekend: Duo handles this complex electropop track with dazzling skill. This song showcases the bass, dynamics, separation, and soundstage all together. Then it reveals how Duo is more than the sum of its parts.

If You Could Read My Mind by Gordon Lightfoot: this 1970 balad is haunting on Duo. Like the U12t, it is both revealing and forgiving of older recordings. Vocal timbre and instrumental texture are pristine. Same descriptors apply to Yesterday and all other Beatles classics. Oldies are golden on Duo.

In Da Club by 50 Cent: The rhythmic bass and surrounding effects make you turn it up and bob your head. You're driving down the road with a 12 inch sub in the back, and audiophile 6×9 speakers handling the details.

Bubbles by Yosi Horikawa: mezmerizing complexity to show off what Duo can do. Think Duo has rolled off treble? Listen to this.️

Conclusion

I don't say this lightly, but Duo is the best sounding IEM I have ever heard. For my taste, it perfectly balances technical prowess, powerful bass, and gorgeous musicality. This is all immersed in a massive soundstage, with no harshness up top. Duo is the stuff of dreams.



Favorite Pairings:

Source: Cayin N7 DAP. This DAP is great for any IEM with a DD. Detailed, yet warm & musical. Has expansive staging effects. The synergy with Duo is

Cable: Ranko Acoustics, pure silver. I think tips typically make a bigger difference with IEMs than cables. But, this cable does somehow both brighten up and smooth out Duo a bit.

Tips about tips: I tried 10 various eartips. My favorite is the same that worked best with the U12t, the comfy and natural sounding Tri Clarion Wide Bore. Wide bore tips are perfect for Duo because they do not obstruct the Tia* driver that is designed for that. With Duo, I don't have to use tips that smooth out the upper mids, such as my usual #1 tip, Penon Liqueur Orange. Also, Clarion's thick silicone walls and excellent seal tighten up the bass. On top of all that, the wide bore opens up the soundstage.

*I later discovered that the JVC Spiral Dot Pro and the Penon LS Tips work just as well.

*Tia stands for "Tubeless in-ear audio" design, meaning the driver is directly exposed to the ear canal without additional tubes or dampeners." - 64 Audio

Overview:

Eeets So Leeetle. I’m kidding, it’s the same size as every other 64Audio IEM. The Duo is the cheapest though at $1,200. It also only has two drivers – and it’s semi-open-back. Wow. It’s also lightweight and won’t cause eardrum pressure thanks to the open…ness. The Duo has a Tia high driver and a Dynamic lower driver. My initial impressions of it weren’t that great but I had just listened to the Mezzo, which is not really in the Duo’s league. So how does it do against other IEMs in its class? Well, I have the Thieaudio Monarch Mk2 (MMk2) and the Empire Ears ESR Mk2 (ESR) on my desk to compare. LET THE BATTLE OF THE TWO’s COMMENCE! (See what I did there?)

Build Quality / Comfort:

The build quality on the Duo is pretty good. but feels a little cheap. The little mesh grill covers are top-notch and feel really sturdy. The shell is comfortable and of good quality, though it feels plasticky (their website doesn’t say what it is - could be plastic or metal.) These look like they won’t scratch easily and they can’t bubble like resin (ESR *Cough*) so they would be great everyday IEMs you wouldn’t have to worry about damaging. I do worry about getting lint and dust inside them with the semi-open ear plates, but it’s probably not a big deal. The case is a nice leather circle (that is easy to open…Aroma…) and the ear tips fit inside it along with some filters and a cleaner. It’s a good presentation, though the foam holds on to the IEMs REALLY hard – careful removing them so you don’t break off a cable pin inside the Duo.

The stock cable on these is actually pretty decent. 4-strand multi-twist with silver plated copper – 26 AWG. Nice. Too bad it’s 3.5mm – but it’s still quite nice compared to some - I’d use it if it was 4.4mm. Instead, I’ll be using my trusty modular Kinnera Leyding for testing because…it’s awesome (you can get one here if you feel like it: https://amzn.to/3l4O2Z1.) Ditto for the ear tips – the ones that come with the Duo are pretty nice and there’s a good selection, but mine was used and they’re covered in…stuff…so I’ll be using my Spinfit W1 tips (also, if you want some of these, they’re here: https://amzn.to/3XXudBt.) I use the small size to get a really good fit (yeah, it sucks you can't just order one size – I don’t need 3.) The comfort level with the Duo is really good; it’s what I would classify as Schmedium – around the size of the ESR – MUCH smaller and more comfortable than the MMk2.



Sound / Source / Comparisons:

Looking at the squig.link frequency response graph, the MMk2, and the Duo are very different in most every measure. The Monarch has a tiny bit of increased sub-bass at 20 Hz – but then the Duo overtakes it HARD until the mid-range where they meet for a second and then careen off in different directions once more with the Duo having much more muted upper-mids and low-highs. It will be interesting to see how different songs play out between these two. I’m not including the ESR in this comparison because it has VERY different tuning (far less bass.)



I will be running the ESR and MMk2 from my Shangling M3 Ultra (M3U) using 4.4mm from Tidal Hi-Fi. I am running the Duo at 27/100 on low gain, which is still quite a bit lower than the MMk2 (35-40/100), but far less than the Vision Ears VE8. The lower volume will save some battery in the long run on mobile devices, but there are more efficient headphones if that’s your thing.

As usual, I don’t like breaking down headphones solely by frequency range since every song has bass, mids, and highs (and I can’t tell the difference between vocals at 1900 Hz and 2100 Hz.) So, I will start with bass-heavy songs, then move to mids-focused and lastly highs-focused songs, then break down each song by how all the pieces are presented. You can find my Tidal test tracks playlist in my signature if you want to compare them to your headphones. As usual, I am going to start with the bass-heavy song by David Guetta, “I’m Good (Blue.)” The intro kick drums have lots of impact without any bloat. The mid-synths sound great and the Sub-bass has that breath-stopping quantity that I look for from this song. These are really bassy IEMs – something we should expect from the freq chart. The soundstage sounds wide and open (not surprising with the semi-open design.) The mids sound further back than on the MMk2 (Which is one of the most mids-forward IEMs I’ve heard.) The Duo manages to avoid the sub-bass bloat that even the excellent Aroma Thunder exhibits on this song. For my preference though, I like the forward mids of the Monarch on this song better (I am not a basshead.) The MMk2 has more bloat/extra reverb on the intro bass drums – back to back, the Monarch’s bass is worse than the Duo. The mids are definitely better/more forward/clearer. So between these two, Duo wins bass with tighter/more forward/controlled/impactful bass, Monarch wins mids with more forward/musical mids, and highs are more pronounced on the MMk2 (whether or not that’s a win is up to you.) But, since this is the bass test song, Duo wins the bass.

For the Mids-test, I am using Peter Schilling’s “Major Tom” (which has to be turned up because it’s quiet on Tidal.) The intro synths come in nicely on the Duo, though a little muddier than they do on the Monarch (once again, the Monarch is world-class on the mids, so there’s bound to be some weakness here.) Still, the guitars and vocals sound really good and the bass drums don’t overwhelm the rest of the song. The transition at 1:20 sounds good and has no technical issues. The song comes in clean with all of the instruments having clear separation with an excellent soundstage. Just like the previous song, the MMk2 emphasizes the mids more – so vocals sound very forward and guitars sound crisp and clean on the Monarch (especially at 4:08 where we get into the high vocals – the best part of the song, the MMk2 does it better but the Duo is no slouch here.) The monarch’s bass drums feel very recessed on this song - the Duo gives them more body. The same goes for the bass guitar which is more emphasized on the Duo. There is an annoying sound in the right ear on this song that the MMk2 makes more obvious than the Duo – I will probably never use this song as a test song again because of that sound (never heard it on speakers before, but IEMs exacerbate it. MMk2 win here.

Moving on to “High Hopes” by Panic! At The Disco to test for sibilant “S” sounds. These sounds come in over 3k Hz, so with the Duo recessed treble it could decrease the sibilance on this song – and it does. This is one of the least sibilant colorations I’ve heard on this song. There is almost no sibilance throughout the song (a little bit, but it’s only noticeable if you’re looking for it – which I am.) The vocals, soundstage, instrument separation, horns, and drums all come in clearly – this is a really good song for the Duo. Of course, the Monarch has forward mids here and the drums and horns (which reach into the low-mids frequency) actually get lost a little bit in the background compared to the Duo. The Monarch sounds sharper in the highs here compared to the Duo, but it still manages to avoid most of the sibilance – there is more than on the Duo if you’re listening to it, but the Monarch still has some of the best highs you’ll find in this price range. Those of you looking for highlighted highs will want to avoid the Duo – those of you who hate sharper/pronounced highs and love more low-end will love the Duo. Duo win

Well, how does the Duo sound with Michelle McLaughlin’s “Across the Burren?” Really good, the soundstage is massive and recessed highs allow this song to present the piano without any of the harshness that some IEMs/Headphones can display here. If you’re worried that the recessed highs mean that the piano won’t come in nicely, you have nothing to worry about – the piano still comes in nicely, though those of you looking for “Sparkle” may dislike the Duo. There is nothing “recessed” about this song on the Duo – it sounds really good – not Mezzo or Thunder good, but really good nonetheless. The Monarch highlights some of the background noise in this song, which can actually be a little annoying. The notes on the piano are beautifully presented though. There is some sharpness that isn’t present with the Duo, but overall, the MMk2 makes the piano sound more beautiful than the Duo (sparkle anyone?) Tie?



Conclusion:

This is a pretty easy conclusion. The Duo are for people who like low-end more than anything else. You can see on the Freq chart that 64Audio put that as their emphasis and it really shows. Great quality/quantity bass is the Duo’s forte, though it has a good soundstage and instrument separation as well. The mids and highs are definitely more recessed, which may be some people’s preference. If you prefer a similar soundstage and mids/highs forward, the MMk2 remains one of the best in its class. The ESR is also a good option in this price range if you want a great-sounding IEM with less bass than either the MMk2 or Duo with more balanced mids and highs.

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The product was provided to me free of charge for the review purpose in exchange for my honest opinion. The review was originally posted on my site, and now I would like to share it with my readers on Head-fi.

Manufacturer website: 64 Audio. Available for sale directly or from different retailers like Audio46.

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Intro.

Early this year when 64 Audio posted a teaser with 3 of their upcoming new IEM models, one of them got extra attention due to a design with an open mesh faceplate grill, leading many to assume a new flagship with a planar magnetic driver. You can’t blame 64 Audio fans because they have been waiting for something brand new for a while now, and considering today’s market trend with prices going up exponentially, some were prepping for another mega-kilobuck release. Part of their wish came true when 64 Audio introduced a brand-new Duo model, and many let out a sigh of relief when the price of this dual driver IEM was announced.

Without sounding too cliché, I honestly had no idea what to expect, didn’t know at all what is inside under the hood of that faceplate when Duo arrived. I like surprises, and this was a pleasant one. Now, after spending close to a month of listening and testing of this latest 64 Audio release, I would like to share what I found.



Unboxing and Accessories.

Not sure if 64 Audio is still in the process of tweaking the design of their universal IEMs packaging since Duo box is somewhere in between of Nio and U18s releases. Here, it looks more compact, similar to Nio, yet the design is more “traditional” with exterior sleeve and magnetic flip cover box underneath, similar to U18s. To get your attention, sleeve cover features a bold look of the mesh cover faceplate, while cover’s back has a CAD drawing of the interior design and key features printed underneath.



Inside you have a big foam block with a cutout for Duo and the cable snaked around it, as well as a round opening for a Premium Leather case with accessories inside. It’s a nice quality leather case, great for secure storage or for a display on your desk. Inside the case you have a plastic organizer for all the included eartips pairs, such as TrueFidelity foam tips (S/M/L), SpinFit CP145 silicone tips (S/M/L), and generic silicone tips (S/M/L). Plus, you get a shirt clip and a cleaning tool, in addition to a round sticker with new 64 Audio label.



Cable.

Since I haven’t tested U6t where it was first introduced, this was the first time I had a chance to see 64 Audio new Premium cable included with Duo. This very low impedance cable, according to 64 Audio, measures 0.23ohms, features 4 separate silver-plated OCC copper wires, each with 26 AWG thickness. 64 Audio refers to this cable as 7x7x4 multi-twist where I just assume ‘4’ stands for a number of conductors.

The cable feels more durable, not as flimsy as previous stock cables. It has a glossy rubbery thicker jacket, loosely braided wires between the plug and y-split, and twisted pairs of wires going to each earpiece. No more dealing with a memory wire, here you have a pre-shaped earhook heat-shrink tube, and premium angled non-recessed 2pin connector housing with L/R letter indicator.



The compact all metal y-split and matching metal chin slider have updated design, and y-slit also features a printed 64 Audio logo. Cable is terminated with a matching metal plug w/64 Audio logo, though the plug itself only offered in single ended 3.5mm by default. In the past, many audiophiles have complained about stock 64 Audio cable. This upgrade with a higher quality hardware and more durable build should make people happy, or at least happier.



Design.

64 Audio is very straight forward with their hybrid IEM naming, Trio – triple driver, Fourte – four drivers, Nio – nine drivers, and following the same logic, Duo – dual driver. So, Duo is a dual driver hybrid design, featuring one 9mm DD low and one tia BA high drivers, along with a built-in apex module/core and LID tech. The shell itself has a traditional 64 Audio shape which is nearly identical to Trio and similar to Fourte, including a bulbous shape of the nozzle base, so expect a similar fit. And similar to those other IEMs, you will find a larger hole drilled at the bottom edge of the shell facing forward - a dynamic driver vent. But you will not find apex module pinhole vent here.



While Nio didn’t have a separate apex vent in the shell due to the module being replaceable, both Trio and Fourte do have this pinhole vent since their apex module is internal to the shell. In Duo description, 64 Audio refers to “Apex Core” where according to the design its faceplate features a perforated grill that leaves the shell of the IEM open to the outside for a less obstructed airflow. The apex module, which stands for air-pressure-exchange, is intended to relieve air pressure in a sealed earcanal, thus you can see a connection relative to this new faceplate design. And while apex module is not replaceable, according to the spec Apex Core system in Duo provides isolation of -12dB.



Actually, the faceplate design is quite complicated and consists of three-piece aluminum assembly. You have an anodized aluminum grill fixture underneath of a black PVD coated stainless steel mesh, all high tolerance design. The mesh is secured to the grill with a chromed stainless-steel bezel. The aluminum shell itself has a solid build, and while appearing black from a distance, it has a very dark navy anodized finish when you look closer. The tip of the nozzle has a protective mesh grill over tia driver to keep wax build up away.

And speaking of tia driver, which is an open BA driver used in majority of 64 Audio iems (usually placed in the nozzle), in Duo design they refer to it as a “complete Tia System to reduce unwanted resonance and distortion by allowing 9mm DD + Tia high driver to radiate into the single large Tia bore with no obstructions to the front of the driver”. If I understand it correctly, the tubeless output of DD driver goes through open BA tia driver where the sound is mixed and comes out of the nozzle and into your earcanal.

Last, but not the least, Duo also features 64 Audio own Linear Impedance Design (LID) where despite a low impedance (9ohm), Duo should sound the same when paired up with different sources, regardless of their output impedance. Just keep in mind, every source has their own sound characteristics which is going to contribute to the final sound you hear based on the synergy between IEMs and the source. But the actual output impedance of the source shouldn’t have the effect on the sound.

And again, I want to mention that while this is supposed to be open sound design with a mesh/grill faceplate and only -12dB of isolation, there is hardly any sound leakage. People standing a few feet away from me couldn’t hear any sound, and I even tried to hold my phone an inch away from Duo in my ears while recording ambient noise and didn’t hear anything.



The fit.



Sound Analysis.

I analyzed Duo sound performance paired up with a neutral LPGT source while playing a variety of test tracks, such as Agnes Obel “The curse”, Sandro Cavazza “So much better” (Avicii remix), C-Bool “Never go away”, Ed Sheeran “Shape of you”, Alan Walker “Darkside”, Galantis “Hunter”, Iggy Azalea “Black widow”, Indila “Boite en argent”, Dua Lipa “Love again”, Counting Crows “Big yellow taxi”, Bob Marley “Jamming”, David Elias “Vision of her”, and Michael Jackson “Dirty Diana”. I let it burn in for at least 150hrs before I started taking my notes.

I would like to start off by mentioning that due to lower sensitivity (98dB), Duo will require higher volume, about 8-9 clicks higher than my other average sensitivity IEMs. It’s not a big deal, still easy to drive. But when you are comparing and switching between different IEMs, don’t make the assumption that Duo will be OK at the same volume level as others. The volume setting here is important, otherwise mids will be more recessed. Push the volume higher which is still OK due to apex core design that relieves any sound pressure.

To my ears, Duo has a distinct U-shaped sound signature with a recessed perception of mids due to elevated bass and mid-treble. The tonality is on a warmer and smoother side with a fuller body sound. I didn’t find the sound to be micro-detailed or analytical, though tia driver helps to bring up the resolution. And despite a warmer fuller body sound around bass and lower mids, the retrieval of details is good with the sound being clear, not muffled or veiled. The overall sound has a relaxed, laidback presentation with a smooth, natural tonality, and a decent level of natural detail retrieval.

The bass has extra weight with a noticeable presence of extended and elevated sub-bass rumble that gives low end analog timbre, and mid-bass impact I associate with floor standing speakers. Bass is relaxed, with average speed attack and decay, typical of DD driver, not too tight or articulate, but with a good control. Sometimes extra decay of bass can contribute to thickness of lower mids body, but not in this case. To my ears, I don't hear bass spilling into lower mids.

Mids are smooth, detailed, natural, with plenty of body, yet without being muddy or veiled. Especially when you focus on vocals, you can appreciate a natural organic tonality of both male and female singing. So, lower mids have more body while upper mids are smoother and more organic. I usually look for sound separation and layering in mids, while here it is just average due to a more natural smoother tonality. Mids are not lacking any details, just less revealing. Also, when it comes to imaging, mids/vocals positioned farther out of your head, and sound a bit distant. You do need to raise the volume to bring up mids quantity, especially due to lower sensitivity of Duo. As I already mentioned, if you are switching between different average sensitivity IEMs and go back to Duo without changing volume, mids will be lacking resolution. But as soon as you turn up the volume, they come alive.

Treble is the same way, need to pay attention to volume setting. Duo is not for low volume listening. Once you raise the volume to a more adequate listening level, the clarity and the sparkle of treble goes up as the tia driver starts to flex its muscle. Here, lower treble is smoother, so don't expect any harsh sibilance peaks. Instead, the high definition and airiness of the treble comes from mid- and upper-treble extension. Treble does pack some energy, but still sounds natural and with good non-fatigue definition.

Despite a more open sound, I wouldn't call soundstage expansion to be super wide. It is wide and definitely above average, but I hear more depth/height than width, with the sound surrounding you with more intimacy. The open sound feeling comes from less isolation and zero sound pressure which makes you feel like you are wearing open back headphones, but with a benefit of hardly any sound leakage. The imaging is good, not super precise due to more depth and less air between layers of sound, so don't expect 3D holographic imaging. But it is pretty convincing, nothing is congested, and it is easy to distinguish and position instruments and vocals.



Eartips selection.

The selection of eartips is crucial to any universal in-ear monitors and will affect the sound, especially the bass impact/quantity which depends on the seal. Due to a large opening of my earcanals, I usually go for the largest size eartips to get a better seal. Also, please keep in mind, eartips impressions are subjective and will be based on and relative to anatomy of my ears.

stock SpinFit CP145 - laid back smoother natural tonality with a more U-shaped sound sig.

stock TrueFidelity Foam tips - laid back smoother natural tonality with a more balanced W-shaped sound sig.

stock Silicone tips - these had a little bit of lift in bass and treble.

Azla Xelastec - more V-shaped sound due to lift in bass and treble.

Final Type E - similar to stock SpinFits.

Symbio F - similar to stock except treble was a bit brighter.

For me personally, stock CP145 and Symbio F were preferred eartips with Duo.



Cable pair up.

I’m aware that some people don’t believe in cables and have very strong opinion about it. It’s not my intent to change those minds. Instead, I’m just sharing what I hear during my testing. What makes sense to me, a metal wire is a material with physical properties of resistivity, conductivity, purity, and unique geometry, all of which put together act as a filter between your source and earphones. Variations of these physical properties can affect the conductivity of analog signal, resulting in a sound change, from a subtle to a more noticeable level. If the talk about cables upsets you, please skip this section. Otherwise, enjoy these short impressions.

stock to 64 Audio Premium Silver hybrid - the main change I hear is in mids, being a little more revealing and noticeably more forward.

stock to Eletech Socrates - very interesting effect of more elevated sub-bass and even stronger mid-bass punch and crisper treble. The bass lift is actually not just subtle, but quite noticeable.

stock to PS Copper+ - another example of bass and mids lift, while treble remained the same. Also, bass feels faster and tighter.

stock to EA EVO10 - in this pair up I hear sound sig being more V-shaped due to a stronger bass and lift in mid-treble.

stock to PWA 1960 2wire - I hear more clarity and resolution in mids/treble, but at the same time, bass is lifted and the tuning becomes a bit distracting.

I actually enjoyed 64 Audio premium silver hybrid cable pair up the most because it added some clarity to the mids and didn't lift the bass like in pair ups with other cables. Stock cable pair up is also good if you want a smoother and more laidback tonality.



Comparison.

The comparison was done using Duo with a stock 3.5mm cable, SpinFit CP145 stock eartips, and LPGT as my source, volume matched in every comparison.

Duo vs 64 Audio Nio - Had to bump Nio to M20 module to come closer to the weight and the impact of Duo's bass. That gives a good match in sub-bass rumble and a similar mid-bass impact. Duo has a fuller body lower mids, while Nio's lower mids are leaner, and as a result Duo's mids sound a bit thicker while Nio's mids (vocals) have a little better definition. This difference in mids was more noticeable with vocals rather than instruments. When it comes to treble, Duo is brighter and has more air with better extension in comparison to Nio. Furthermore, the soundstage is more open in Duo, especially since I'm using M20 (20dB) apex with Nio which has more isolation.

Is Nio worth extra $500? IMHO, yes, considering you have replaceable apex module and prettier blue abalone faceplate. But if you don't care about it and prefer more open sound and authorative bass impact, then Duo would be a good choice. But also keep in mind, Nio will have better isolation while Duo by design is not.

Duo vs 64 Audio Trio - The sound signature is the first thing that stands out when you start comparing these iems, both do have recessed mids, but Trio is V-shaped with a deeper cut in mids while Duo is closer to U-shaped. Both have a very similar bass down to a deeper sub-bass extension and punchy mid-bass. Duo's lower mids have more body which gives its mids/vocals more organic tonality, while Trio's lower mids are very lean which translates into a thinner and brighter mids/vocals. Treble is another extreme with Duo being smoother and more natural while Trio treble being a lot more brighter and piercing. Soundstage expansion is actually not too far off, but Duo does give you a more open sound perception.

Duo vs 64 Audio U12t - I was using U12t w/M15, but its soundstage is still a little narrower in comparison to Duo. The overall sound sig of U12t is more balanced while Duo has a U-shaped signature. Duo has more elevated bass, definitely more weight with deeper rumble and stronger mid-bass, making U12t sound leaner and even more neutral in a relative comparison. Mids of U12t are leaner as well because of less body in lower mids, while Duo has thicker and warmer mids with fuller and richer tonality body. And while both have a clear detailed treble, U12t has more sparkle in comparison to a more natural tonality of Duo's treble. There is definitely a noticeable difference in weight between DD and BA bass, but what stood out the most for me was mids/vocals. If you want a lusher fuller body tonality, Duo is a great choice, while U12t delivers leaner and more detailed mids/vocals.

Duo vs CFA Solaris 2020 - Gotta remember to adjust the volume in this comparison, due to Sensitivity difference I had to lower the volume by about 17 clicks when switching to Solaris. I few people asked me to include this comparison, though these are quite different in every aspect. Solaris soundstage is wider with a more holographic imaging, while Duo does sound open but soundstage has more depth/height than width. Overall tonality and signature quite different as well, with Duo being warmer, smoother, and U-shaped, while Solaris being brighter, more revealing, and balanced with more mid-forward emphasis. Duo's bass definitely scales up in weight with sub-bass rumble being more elevated and digging in deeper, and with a stronger mid-bass impact, making Solaris bass sound almost neutral in comparison. With mids, Duo has more body and warmer tonality in comparison to leaner body and brighter and more revealing upper mids of Solaris. And the same with treble, Duo being clear and more natural, while Solaris being brighter and harsher in comparison to Duo. So, again, if you want a more revealing micro-detailed holographic sound, Solaris is a better choice here, while Duo delivers and more natural, detailed, smoother sound with a more analog bass and more intimate presentation of the sound.



Source pair up.

In each source pair ups, I was using a stock 3.5mm cable and stock SpinFit eartips. Duo is not that hard to drive, but considering low sensitivity of 98dB and 9ohm impedance, I had to push volume 8-9 clicks higher relative to my other IEMs. No hissing was detected. For your reference, these are very brief pair up notes. And by brief, I just focus on any changes related to signature and general tonality, without going into too many details about technical performance difference.

Lotoo LPGT - laidback smoother natural tonality with a U-shaped sound sig (baseline).

iBasso DX300 MAX SS - stronger bass (sub-bass and mid-bass) impact with a little brighter mids.

Cayin N6ii w/R01 - similar to 3Max SS, a stronger bass and a little brighter mids.

Hiby R6 2020 - similar sound presentation as LPGT, smooth and natural.

L&P P6 Pro - similar to LPGT but with a bigger soundstage expansion and more analog tonality, along with improvement in retrieval of details.

A&K SP2000 SS - nice articulate bass and detailed natural mids, but treble in this pair up is a bit too bright and shouty.

Lotoo LPGT and L&P P6 Pro were my favorite pair ups with Duo.



Conclusion.

As I was thinking about what to write in the conclusion of this review, I revisited my U18s write up from March of this year, and realized that I also offered my speculation about the picture of an open back grill IEM, keeping my fingers crossed for the next big flagship release. Maybe, this is just a part of our audiophile nature to wish for the next big thing. But in reality, we get excited just to hear the next “new” thing, especially from established brand like 64 Audio. And at $1,199, Duo is actually the cheapest universal hybrid IEM from 64 Audio that features all their signature tech, including tia, apex, and LID.

But what gets many excited about the hybrid design is usually a dynamic driver, and thanks to its 9mm bass DD, Duo can definitely satisfy a craving for that analog speaker like bass paired with a clear detailed tonality and extra sparkle of upper frequencies delivered by its tia driver. The natural, detailed tonality of this IEM has a non-fatigue relaxed tuning. And the open sound design with less isolation and minimum sound leakage, unlike typical open backs, makes Duo great for use in public places if you don’t need total sound isolation and instead prefer to keep awareness of your surroundings.

Actually, it doesn’t matter where you use Duo, indoors or outdoors, in public or in private. What matters is the laidback relaxed tuning and non-fatigue tonality with deep lows and clear detailed highs you can enjoy anywhere. Now, having said that, 64 Audio, we are still waiting for the next big flagship!