64 Audio Duo Review

While most of 64 Audio’s Universal IEMs have their roots in custom fit designs, made for the stage and studio, they’ve consistently had a few models aimed squarely at audiophiles. Starting with the Fourte, 64 Audio used the same technology as their other IEMs, but dropped the driver counts down and tuned the IEM more for musical enjoyment, than for studio monitoring. Fourte was a four driver hybrid, followed by Trio which was a three driver hybrid. Now Duo is here as a – you guessed it – two driver hybrid. Duo brings unique feature as well: it’s a totally open-back design in an IEM. Let’s take a closer look to see how Duo fits in with the rest of 64 Audio’s IEMs.

Build and Design

Duo has a unique look to it, which shows 64 Audio branching out in a number of ways. Dubbed “Apex Core,” its open vented design consists of a thin, perforated faceplate which is also the central aesthetic feature of the IEM. The included cable is the new model first included with the U6t, and still represents a big improvement over past 64 Audio stock cables in terms of perceived quality, feel, and manageability.

64 Audio Duo

The rest of the package is nicely put together, with the updated eartip selection, including some SpinFits specially designed for 64 Audio, and the requisite leather-wrapped carrying case. The general aesthetics, build quality, and presentation are all quite good and meet expectations for the $1199 price point.


While it is open back, the bleed isn’t nearly on a level with open-back circumaural headphones either in or out. We had these on the show floor at CanJam SoCal and didn’t have any negative feedback about outside noise interfering with the listening, and in our (mostly) quiet office, at moderate volumes, these were more comparable to a closed back headphone than any open-back.

The Sound

Sometimes you put on headphones, and you need to sit and listen for a little while to pick up on the specific characteristics that make it unique. Other times you turn on a song and are immediately hit with a few standout characteristics. Duo is the latter, and the moment you plug it in, the immensely wide soundstage begins to take shape. Not long after that, you catch on to its incredible resolution.

The tuning is generally V shaped, though not in an exaggerated way. There’s good subbass extension and a tight punch in the midbass. Duo has good separation between the mids and bass with minimal bass bleed, though the bass isn’t very strongly textured. The mids are well constructed, but vocals and guitars feel just slightly behind in the mix, particularly with rock and heavier music. 

64 Audio Duo

The upper mids into the treble have a smooth presentation that leans more towards a relaxed sound, though potentially at the cost of some forward presence in higher ranged vocals and instruments. The treble demonstrates a lot of air, with good delivery of upper range percussion, and a clean, crisp attack and smooth decay on instruments like cymbals and higher parts on strings, with a good sense of breath on wind instruments as well.

The overall sound feels balanced and natural. Drums in particular stand out as being very well executed, while the instrument timbre is just right. The soundstage is wide and deep – with a very strong “outside your head” feeling. Sometimes it does feel slightly exaggerated between the furthest left/right and center. With the imaging there’s a sensation of an almost W shaped stage with good three-dimensionality, but with a slightly wider than expected gap between the instruments on the left and right and the center. The positioning as a whole is strong, with a generally holographic feeling.

On Kings of Leon’s “Use Somebody” the opening vocals of the verse are up close with a personal emotional feeling. The guitar sounds rich and raw flanking the singer. On the instrumental builds of the intro and between verses, the bass takes center stage, with guitars on either side, and the drums behind them. Cymbals crash to the left and right, with the bass and snare hitting right down the middle. The bass has a rounded feeling, without having a strong texture to it, but the overall sound is very detailed.

64 Audio Duo

The opening piano on Lauryn Hill’s “Doo Wop (That Thing)” hits with a delicate stacatto rhythm, but is quickly moved back by the impact of the bass and drums. While Duo puts the accent on the tight punch of the blend between the bass and kick drum, the piano and other elements remain well placed in the mix. The vocals are clear and detailed, with good separation between the various vocal parts and good distinction between the voices. Duo also demonstrates strong, powerful, impactful bass that doesn’t bleed up into the mids.

Duo gives a thick, liquidy texture to the opening notes of “Lingus” by Snarky Puppy. “Lingus” is the sort of song that gives every aspect of a headphone a workout. Can the subbass properly deliver the deep synth bass? Yes. Can you hear the intricacies of the fast cymbal work? Also yes. How is the timbre on the diverse instruments, ranging from bass guitar, electric, saxophone, and trumpet to various keyboards? Bass guitar is warm and smooth, guitar has just the right amount of bite, horns are clear and balanced – not too smooth, not too sharp – and each synth and keyboard sound has a unique textured sound. Throughout Duo demonstrates strong instrument separation and good detail and tonality with each instrument.

“White Rabbit” by Jefferson Airplane is a slow build of a song with a bolero-like feel to it. The slow build allows the Duo to place each instrument with weight and precision so that as it reaches its climax you have a clear, well-defined 3D image. Grace Slick’s voice is haunting and almost ethereal – which is quite appropriate for Jefferson Airplane’s brand of psychedelic rock. With Duo’s wide soundstage, there’s a sense, as the band builds to its final crescendo, of everything closing in and pushing forward to completely engulf the listener.

Comparison: Audeze LCD-i3

While there have been a handful of other open-back earphones, probably the most notable is Audeze’s iSINE and LCD-i series. Where Duo is simply an IEM design with an open, vented faceplate, the Audeze’s take on the open-back in-ear is almost like they shrunk one of their LCD-series open back headphones down to fit in your ears.The uniquely designed LCD-i3, priced at $899, provides an interesting headphone comparison to the more conventional Duo.

64 Audio Duo

In terms of the overall design, LCD-i3 looks cool, but it’s a little more obtrusive than the sleek Duo. LCD-i3 definitely sacrifices some comfort for the sake of performance and the general concept of an in-ear planar magnetic headphone. I have a really hard time getting a comfortable, stable fit with the LCD-i3, even with the provided earhooks, while Duo has a light, easy, comfortable fit. LCD-i3 also has more open back characteristics in general, including a bit more noise bleed in and out.

In terms of the tuning, Duo clearly has more emphasis in the bass, while the LCD-i3 feels a bit mid forward, without the same level of slam. Both have a similar sense of air, and strong performance with cymbals and the higher range of percussion. The timbre of LCD-i3 isn’t as strong as Duo, with a less natural delivery of acoustic instruments and strings. In terms of overall tuning, LCD-i3 has some strong characteristics, but Duo has a more cohesive overall tuning.

Both have excellent soundstages, and Duo’s surprisingly feels a bit wider, but the LCD-i3 has a more rounded, consistent soundstage. The planar drivers in LCD-i3 feel a little faster, and combined with the stronger mids have some edge in the technical delivery of more complicated music, like the Snarky Puppy track mentioned above. But the slightly smoother delivery of the Duo feels stronger on tracks like “White Rabbit” or “Doo Wop (That Thing)” where LCD-i3 delivers all the notes with aplomb, but doesn’t foster the same sense of connection between the listener and the music. 

Overall, they’re both excellent options, but Duo presents a more refined, accessible take on the open back in-ear concept. If you can get past the usability issues with the LCD-i3, it presents a great package, with strong technical capabilities, but one that still feels like more of a niche product.

Comparison: 64 Audio U6t

Looking at 64 Audio’s lineup, you may notice that the U6t and Duo are quite close in price. When you look at the driver counts, you might also ask why $1299 gets you six drivers while $1199 only gets you two. In the end, it comes down to U6t and Duo being made with different goals for different preferences, but let’s take a closer look at the specifics.

64 Audio Duo

Starting with the basic design, U6t is a traditional IEM with only the small Apex vent, while Duo is open. Duo is also a hybrid design vs. the fully balanced armature based U6t. This means that while U6t has a solid soundstage, it has nowhere near the insane width of Duo. Likewise, Duo produces much more impact in the low end with a stronger emphasis towards slam and macrodynamics, while U6t has a smoother, sometimes more precise sounding bass response.

In terms of the tuning, Duo has a stronger V-shape with more emphasis in the bass, and also more air and sparkle at the top end. U6t’s tuning is more balanced throughout with less bass emphasis and stronger midrange. While there are clear differences in the sound, there’s hint of common DNA between the two. Duo feels like a bolder, brasher take on 64 Audio’s “house sound” epitomized in the U12t or U18s, while U6t has is more like a scaled down version of those IEMs. Basically, if you’re looking for a tuning that’s just on the “romantic” side of neutral, with a focus on precision and accuracy, the U6t will be more up your alley. If you want an IEM with a more exaggerated, fun sound, and a really amazing performance in its own right, Duo is what you’re looking for.

The Bottom Line

While its overall performance meets expectations for its $1199, the width of Duo’s soundstage exceeds just about any conventional IEM at any price point. Of course, sometimes when a headphone comes out with one exceptional feature, deeper dives and longer listening sessions reveal that it comes up short in other ways. Without the whole sound, the massive soundstage would be all icing and no cake. Not so with Duo: Duo has an addictive, imminently listenable tuning with strong overall performance that gives you a delicious cake covered in exquisite icing.