Dynamic, Fun, and Technically Strong | 64 Audio U4s Review

Dynamic, Fun, and Technically Strong | 64 Audio U4s Review

64 Audio is probably best known for the way that they craft tunings that meet the needs of musicians – both on stage and in the studio – and give audiophiles the blend of musicality and detail that they crave. Most of their universal IEM line-up started as custom-only IEMs designed for musicians, and eventually transitioned into a universal model. U4s is no different, taking the 4-driver hybrid design of the A4s, putting it into a universal shell, and hitting at their lowest price yet for a universal IEM. Does U4s deliver the level of quality we’ve come to expect from 64 Audio at $1099?

Build and Design

U4s sticks to 64 Audio’s classic anodized aluminum for the majority of the shell, and it has a dark gray pearloid decoration on the faceplate. The overall design is similar to the U6t, Nio, and Fourte, which all have the same basic visual template, with different colors and patterns. This means you get the same sort of comfort and durability you expect from 64 Audio, with that extra touch of visual flair.

64 Audio U4s Review

The box is pretty standard, and you’ll find the IEMs, 3.5mm cable, case, eartips, cleaning tool, and shirt clip inside, along with 64 Audio’s Apex Modules. Apex Modules provide varying levels of venting for the IEMs which in turn impacts the tuning, particularly in the bass and soundstage. U4s has the m15 module installed out of the box, and includes the more open mX, and tighter, more closed m20 module. We’ll talk more about the practical impacts of those in just a minute.

In terms of using U4s, it’s lightweight and provides a very easy fit. You should be able to find a solid fit with the included SpinFit, wide-bore silicone, and foam eartips (though I could have probably gone for an XL in the wide-bore). In addition to their sonic impact, the Apex Modules also help reduce fatigue by venting the sound, and give you options as to how much air (and subsequently noise) you wanted vented.


My first impression of U4s was that it had a character similar to the U12t, but with the benefit of dynamic driver bass. With just a little more bass than the U12t, the tuning is very close to the Harman-esque “bass boosted neutral” that has become one of the most popular IEM tunings as of late. However, U4s sets itself apart from the pack, particularly in the quality of the bass delivery and the excellent air and resolution in the upper treble.

64 Audio U4s Review

In the low-end, you get a strong emphasis, with deep extension, and just enough extra midbass to push things towards “basshead” territory. The texture is excellent, offering bass that doesn’t just hit hard, it also provides a clear tight presentation. 

The midrange provides excellent detail and a good weight to vocals and instruments. In the overall tonal balance, the midrange is definitely a step back from the bass and treble, but it doesn’t lack detail or vocal presence at any point. The treble is a surprising highlight here, lending definition and clarity to the sound, and offering excellent air and sparkle at the top, with a good sense of resolution as well.

U4s has strong imaging characteristics as well. The soundstage is reasonably wide and three-dimensional, though not expansive.The positioning and separation between instruments is strong, and vocals stand out nicely. The imaging is probably the only piece here that’s very good, but isn’t absolutely incredible for the price.

From the imaging and soundstage to the bass response and midrange, you actually have a lot of options to help tweak the sound yourself. The Apex Modules in particular offer an array of options to either expand the sound and enhance the imaging, or tighten things up and accentuate the bass and dynamics. m15 is the default sound we’ve used as a reference so far, but m20 brings in the soundstage a bit, but also adds a harder hitting bass, and brings the low mids up a bit as well. mX opens it up, and offers some enhancement to the soundstage, makes the bass a bit more neutral, and enhances the sense of separation in the image.

64 Audio U4s Review

64 Audio also selects the eartips to bring out different characteristics of the IEMs. The SpinFits (my personal pick, and what I used for my impressions) provide a balanced sound and secure fit, with perhaps the slightest attenuation to treble peaks. The wide-bore silicone tips provide a little more air and a slightly brighter sound. Paired with the mX, the wide-bore tips can almost transform U4s into a totally different IEM, with a leaner bass and more airy, wide open sound. The foam tips restrict the soundstage a bit while providing stronger attenuation of the treble. 

How does U4s do with ironic dancehall style bangers performed by art-rock bands? If that question seems suspiciously specific, it’s just because I wanted an excuse to listen to “Compliance” by Muse again. There are so many diverse elements, from Matt Belamy’s theatrical vocals, to the waves of synthesizers, to a bassline that weaves in and out of the melody, and a driving beat. U4s provides excellent instrumental layering, incredible texture across the full spectrum. There’s a hard punch to the bass drum, but also great clarity in the constantly moving bassline. Elements like the echo-y guitars help define a wide soundstage, while synthesizers stab you from behind in an immersive display of three dimensional imaging.

U4s does an excellent job capturing the vocal dynamics and emotion in Eva Cassidy’s version of “What a Wonderful World.” From the subtle vibrato, to the touch of grit in some of the bluesier vocal runs, you get a full, intimate vocal performance. The bass and kick drum have a tight sense of blending, and get a clear emphasis in the instrumental mix without overpowering the vocals or other instruments. There’s also a strong delivery of the layering between piano, guitar, and vocals, with each shifting to the forefront when appropriate with the ebb and flow of the song’s dynamics.

64 Audio U4s Review

Probably the best imaging I heard from U4s was listening to “Stable Mates” of the Herbie Hancok Trio with Ron Carter album. Each piece of the drum kit was incredibly well defined, and bits of dynamics – like the drummer hitting the ride cymbal harder as the band built up – were reflected not just in the volume of the sound, but in the amount of space the instruments occupy in the image. The upright bass has a great texture and tactile sense of pluck, while the piano has an intense clarity and a vivid presentation of the dynamics of Hancock’s right hand playing in particular.

Comparison: 64 Audio U6t, Campfire Andromeda Emerald Sea

Previously on “IEMs that seem a lot like a scaled down U12t” we had the U6t, so we wanted to see how U4s and U6t matched up in 64 Audio’s lineup. In addition, Campfire Audio just released their latest revision of the Andromeda, a long time reigning champ in the $1000 price range (though Emerald Sea is $1450), making it a good comparison for U4s.

64 Audio U4s and U6t Comparison Review

In the bass, Andromeda Emerald Sea provides a surprising amount of power, with good impact and texture as well. U4s has a little more focus to the impact, but Andromeda might have a little more total bass quantity. U6t is more reserved (also using the m15 and SpinFits as the reference), offering a slight elevation in the bass, but without as obvious an emphasis and also a little more roll-off in the subbass.

Andromeda Emerald Sea doesn’t have as much midrange presence as its predecessors in the Andromeda line, but still offers good upper mids and vocal delivery. U4s and U6t are very similar to each other, with a more rich, complete midrange. U4s surprises in the treble, with perhaps the best sense of air and sparkle of the three, while U6t is close, and Andromeda rolls off some of the top-end.

In the stereo image, U6t had the most three-dimensional feeling, while Andromeda offered probably the strongest sense of instrumental separation. U4s certainly has some good characteristics here, but didn’t have the same width in the soundstage or as much clarity in the imaging as U6t and Andromeda.

64 Audio U4s and Campfire Audio Andromeda Comparison

Basically, U4s compares very well with U6t, but is a bit more fun, while also not being as strong from a technical perspective. Andromeda offers a different take on a similar idea, with a bit more of a v-shaped sound than U4s, and some strong technical capabilities along with it. Either way, U4s is definitely hugely competitive in its price range.

Final Thoughts

Offering an elegant, durable, ergonomic design, solid technology, and a classic sound signature, U4s doesn’t really do very much that’s different from the 64 Audio universal IEMs that have come before it. What it does do differently is – in a world where everything is getting more expensive – offer a taste of 64 Audio’s signature sound quality and engineering at a new lower price. With the level of quality that 64 Audio has delivered here, U4s is looking a lot like the new king of its $1099 price range.