Bellos Audio X4: An Engineer’s Perspective – A Custom In-Ear Monitor Review


Bellos call the X4 a warm reference on their site, and I think that’s the most apt description you could give it. It’s a relaxed, laidback, unexaggerated monitor that delivers music as if it’s its day job; blasé, measured and uncoloured. That isn’t to say it’s unmusical or flat, though. If the track is dynamic, so is the X4. If it sweetly, softly caresses, the monitor will too. As a playback device, it won’t rob music of soul. It just won’t add anything that isn’t there. The same goes for clarity or brilliance; never sharper, more bite-y or more contrast-y than the track needs it to be. The X4’s job is to not flatter recordings; ideal for an engineer. It’s the IEM where I’ve heard my EQs come through most clearly. I can hear the subtlest shifts 1–2dB nudges make. The few exceptions I’d note are its weighted bass and a touch of top-end smoothing. Both are tiny colourations akin to leaning away from your near-field monitors a tad. But, otherwise, it’s a plain canvas that lets you build tracks with minimal bias and little-to-no flattery.

Imaging is where Bellos have chiefly succeeded with the X4. The in-ear’s capable of convincing surround sound with genuine depth on both y and x axes. Height is its least noteworthy axis, yet still, at the very least, it matches that of its peers. Because of how equidistant its notes are from you, it’s easy to tell when an instrument is sitting too far forward or back. This is true even at the diagonals and extremes. Notes panned 45-90° are drawn with great clarity. So, when I’m mixing 16-person choirs panned from far-left to far-right, I know exactly who to bring up or down, based on where they are along the x-axis. In stage size, the X4’s again imitates that of the track. Older records like Michael Jackson’s Thriller or Led Zeppelin’s Presence are aptly wide with notes far apart. Whereas, newer, more compressed ones like Gallant’s Ology or John Mayer’s Sob Rock are more bunched-up and in-your-face. Still, in either case, notes are nicely outlined with a dark backdrop strongly aiding separation. I think this is where the X4 punches furthest above its price tag, and where even the most seasoned of engineers will see its merit.


The X4’s DD-driven low-end is versatile; capable of sitting and supporting the track when told to, and delivering solid thumps when asked. The difference in kick between a tune like Yayennings’ Prescott and FKJ’s Better Give U Up is massive (as their respective engineers intended), and it complies with each near-perfectly. I should say it might overstep neutral for engineers used to a Focal, Avantone or Genelec, but I reckon it’ll sound even-keeled for most. The sub-region that’s specially a tad warm is the mid-bass; around 100-200Hz. There’s a bit more of it than sub-bass, so the timbre of the X4’s bottom-end is on the wetter, more pillowy side. Again, it’ll more closely resemble a near-field pair, rather than a far-field pair with a floor-vibrating woofer. But, that isn’t to say its sub-bass isn’t palpable either. The monitor extends nicely, and there’s no mistaking its woofer for anything but a DD. So, while it isn’t the strictest in the world, the X4’s slight tolerance for warmth neither hurts its ability to adapt from track to track, nor hinders its sub-bass kick.

In fact, the tactile nature of its low-end is key when mixing and mastering on the X4’s. One thing I struggled with on some of my previous work in-ears (which I talk about later on) is the ability to feel the bass, especially in live scenarios, where background noise is rife, and the PA system is rumbling the floor under you. The X4 is the first mixing IEM I’ve used with a DD, and it’s also the first to avoid this problem. Rather than hearing for the bass – leveling it where I hear enough of it – I feel for it instead; leveling it where I have enough thump. It’s a method I’ve found precise and reliable, and it’s gotten praise from some of my peers too. Even at home, where everything is still, I use this technique when my ears fatigue. It could just be me, but I tend to lose track of the lows when I start to tire. Feeling for subs vs. hearing for mid-bass has proven an effective, even superior, alternative in those scenarios, which the X4 makes quite easy.


The X4’s mids are where audiophiles may find it least flattering, but it’s exactly where it endears me most as an engineer. Versus richer monitors that fatten vocals, or brighter ones that sharpen them, the X4’s may seem blase; unbothered. But, once you play a few records through it, you’ll find it simply gives what’s asked of it. On Dua Lipa’s Future Nostalgia, the X4 discerns between her fuller timbre on the title track, versus her airier one on Don’t Start Now. As an engineer who regularly mixes up to 4 keyboards and 3 guitarists simultaneously, it makes each sound easy to distinguish and, subsequently, treat. This applies to positioning as well. How far forward Justin Stanton’s trumpet solo is on Snarky Puppy’s Shofukan, for example, is determined by how loud he’s playing at that moment. It’s that dynamic and tonal malleability that makes the X4 an ideal EQ and mixing barometer. Though, again, not the sultriest of songstresses, the X4’s stoic vocal delivery is what makes it a reliable tool; an engineer’s harshest, yet most reliable critic.

Technically, I think the X4 does enough in the midrange to suffice the engineer. Midrange notes are well-separated, they possess good dynamic range when called for, and details are resolved impressively well for an in-ear of its price range. For me, where it falls short of pricier IEMs is in tangibility or physicality. The X4 lacks that final bit of weight to convince you that the instrument you’re listening to is actually there in front of you. Its notes are what I’d call 2.5D, rather than 3D. To be fair, this is more of an issue for the immersion-seeking audiophile than the truth-seeking engineer. In fact, that lightness makes it easy to spot when an instrument is too brassy or shrill. Still, to make this a fair review, it’s worth making note of nevertheless. Again, though, the X4 is not short of skills to aid the engineer. Clean note placement makes panning a breeze, and the resolution it’s got makes spotting recording mistakes just as easy. Again, as long as you’re in it more for objectivity than engagement, this is a midrange that should serve you well.


Treble’s trickiest in a mixing IEM to me. One false peak, and every snare is metal. One false dip, and every drummer’s playing a sand hat. It’s a tough line to walk, which is why what Bellos have achieved here deserves a lot of praise. In timbre, I’d say it runs the littlest bit smooth. Edges are ever-so-lightly feathered – tapered – and that’s where the warm in warm reference is partly from. Tonally, though, the chameleon-like, track-dependent nature I’ve described above is there again. Listening to tracks like Snarky Puppy’s Take It! with three drummers playing in turns, the X4 ably shows the different degrees of brightness between their hi-hats, cymbals and snares. Again, this isn’t the audiophile’s top-end where brilliance, clarity and shimmer are turned up for enjoyment’s sake. These are what-you-hear-is-what-you-get highs where, more often than not, notes are on the verge of either being too warm or too bright, depending on the record. And, that’s precisely what you want if you wanna know whether your record’s edging warm or bright.

Technically, I feel the X4 is most impressive here; up high. Ample treble extension is responsible for the wide, open, convincing stereo imaging I described in Presentation. And, it’s what gives it its wide dynamic range and separation as well. Notes rarely smear on the X4, and they’re on the quicker side too, so catching mistakes while editing or noticing errors on the final pass are made rather easy. Unlike most in-ears I’ve heard in its price tier, the X4 is rich in headroom, giving you a macro view of your mix, so it’s easy to tell when you’re pushing your limiter too hard, i.e. when the image gets bunched up, and you get that pumping sensation. Where the X4 punches least above its price would probably be in background blackness. Though it surpasses the needs of the pro, in my opinion, and provides ample separation, it won’t outperform pricier mixing in-ears like, say, JH Audio’s Sharona, which I’ll compare it against later on. It’s what creates that 2.5D feeling I described in Midrange. I feel like it’s simply a limitation of the driver count, and I’m sure Bellos are capable of improving on it should they choose to develop a more complex successor.

Stage Use: Live Mixing

I mix front-of-house live at the Tabernacle Family Church three times a week on a Yamaha CL5 console, consisting of 6 channels of percussion, 14 channels of drums, 8 channels of vocals, 8 channels of keyboards, 4 channels of guitars and 1 channel of bass. For special occasions, we typically add another 9 channels of vocals, 1-2 auxiliary instruments like the saxophone or violin, and 2-4 channels of pre-recorded playback.

To mix all those sounds, my in-ears of choice were 64 Audio’s A18t and A18s, then JH Audio’s Sharona. Subbing in the X4 marries a lot of what those in–ears each did best. I moved onto the Sharona from the A18s, because I felt it had more space and headroom to handle the plethora of instruments in the mix. The X4 does the same. It positions everything equidistantly, so you’re getting an accurate, almost-surround-sound view of the mix. But, it does so with a more neutral tone than the Sharona; more even mids, and a less-lifted bass. I usually prefer bass-lifted IEMs for mixing to stave off fatigue, but the DD in the X4 compensates for this effectively. I now feel for the bass, rather than hear for it, which is a much less tiring thing to do. As soon as that airy thump of the DD sits with the lead instrument, then I know I’ve leveled the low-end to my liking.

Besides raw tuning, there are several quality-of–life advantages to the X4’s design as well. The ATOM modules allow for both spaciousness and pressure dissipation, which effectively stave off fatigue. Not only can I mix for longer periods, but I’m a lot more relaxed when I do so too. Bellos have also nailed the sensitivity of these X4’s to me. Whether it’s my Yamaha CL5 console or my Aviom personal monitoring mixer, these in-ears are impedant enough to not pick up too much of their noise floors, but they’re sensitive enough to have a decent amount of play on both devices’ volume knobs. Finally, because of the X4’s shorter, aperture-sealing nozzle, putting them in and taking them out is much easier to do than my full-acrylic monitors, so I get to compare the in-ear mix to how the room sounds in a much quicker manner.

Stage Use: Drumming

To drum, I’ve mainly used the same three monitors I mentioned in the section above, but I much preferred the A18t over the others, because its brighter treble allowed me to monitor my hi-hats and ride cymbal more accurately. The same goes for the X4 here. On a crowded stage like ours, where everyone has a floor monitor and they’re all bashing together, it can lack the treble to cut through all that noise. That’s not to the in-ear’s discredit at all, though. It needs that transparent tone for mixing and mastering. It’s just that, on the rowdiest of stages, brighter, more aggressive in-ears might be more fitting for a drummer.

On a quieter stage (where everyone else is wearing in-ears too) or the studio, though, the X4’s will work brilliantly. That DD, again, allows me to feel for the kick drum, the space and pressure relief ATOM provides allow me to relax when performing, and the In-Air Canals are a godsend for multi-hour-long practice or recording sessions.

Studio Use: Mixing and Mastering

For studio mixing and mastering, the X4’s are one of the best near-field monitor substitutes I’ve tried yet. They provide an even-keeled, wrap-around-the-head view of the mix, and nothing (bar the previously-noted colourations) ever steps out that line. Beyond the neutral, blank-canvas frequency response we’ve talked at length about, it’s the X4’s wide, evenly-spaced headspace, measured, accurate dynamics and effortless resolution that make it brilliant to build tracks with. Without being overly clinical or transiently soft, it allows detail to come through with minimal bias. So, whether you’re mixing on the road or working at home, the X4’s will do any engineer a world of good.

Again, the quality-of-life advantages of the X4 improve the mixing experience as well. As I said on the first page, I’ve pulled all-nighters mixing and mastering on these in-ears with barely any discomfort, both on the skin, because of its soft In-Air Canals, and in the ear, because of ATOM. And, whether it’s my studio console, my desktop amp or my laptop’s headphone output, the X4 works fine – hiss-free – with each one of them all.



Picture of Deezel


Church-boy by day and audio-obsessee by night, Daniel Lesmana’s world revolves around the rhythms and melodies we lovingly call: Music. When he’s not behind a console mixing live for a congregation of thousands, engineering records in a studio environment, or making noise behind a drum set, you’ll find him on his laptop analysing audio gear with fervor and glee. Now a specialist in custom IEMs, cables and full-sized headphones, he’s looking to bring his unique sensibilities - as both an enthusiast and a professional - into the reviewer’s space; a place where no man has gone before.


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