Bellos Audio X4 Review – Headfonia Reviews

Today, we review the Bellos Audio X4 Hybrid Semi-Custom In-Ear Monitors from Bellos Audio. It’s available now for $999 USD.


Disclaimer: The Bellos X4 was provided by Bellos Audio for the purpose of this review at no cost. As always, the article reflects my unbiased opinion. We thank Bellos Audio for the review opportunity.

Bellos Audio

Few names resonate with the innovation and dedication that Bellos Audio brings to the vast expanse of the pro audio industry. Founded by the dynamic duo of Vlad and Bogdan Belonozhko, also known as the Bellos Brothers, the Bellos name represents over two decades of combined expertise in creating both personal and professional audio equipment.

More than just another foray into the audio market, the creation of Bellos Audio in partnership with FIR Audio was a response to the growing demand for audio products. 

Bellos Audio was created to overcome the apparent challenges that exist in the field of pro audio. The brothers set out to dissect these challenges and get to the heart of the matter, drawing on their combined experience. And in doing so, they dared to discard conventional wisdom in favor of a clean slate.

This bold move allowed them to reimagine solutions and adopt a radically different approach that set them apart in the industry.

The X Series

Bellos Audio’s X-Series consists of three models: X2, X3, and X4. Each model integrates the brand’s signature technologies: In-Air Canals, ATOM Venting, and the RCX Connector.

  • In-Air Canal Technology: Traditional Custom In-Ear Monitors (CIEMs) extend past the ear’s second bend, often causing discomfort and seal breakage during facial movements like talking. Bellos Audio’s In-Air Canal technology addresses this by creating a seal right at the ear canal’s entrance. This design ensures a snug fit, delivering unparalleled comfort compared to traditional CIEMs.
  • ATOM Venting: Beyond just sound quality, the listening experience is about comfort. The ATOM Venting system is designed to minimize listener fatigue by venting built-up air pressures in a sealed ear canal, enhancing the soundstage and spaciousness.
  • 3D Printed Matte Shells: Crafted with patented 3D printing technology, these CIEMs offer precision and a matte finish, ensuring a comfortable, consistent seal.
  • RCX Connector: Addressing a common weak point in CIEMs, the cable, the RCX Connector is designed for durability, rated for 5,000 connections.
  • Removable Faceplate Design: Unlike conventional CIEMs that need to be cut open for repairs, the X-Series features a faceplate attached with alloy steel screws, allowing for efficient repairs without damaging the CIEM shell.
  • Modular Drivers: The X-Series introduces a faceplate that’s attached with durable alloy steel screws. This design innovation allows for efficient repairs without the invasive process of cutting open the CIEM shell.

In essence, Bellos Audio is revolutionizing the design and production of CIEMs with significant quality-of-life innovations that optimize the build and repair process while improving sound fidelity, comfort, and product longevity. I admire their approach, and this is the kind of engineering that we need to see more in this ever-expanding industry.

Bellos Audio X4

The Bellos Audio X4 is the star of the today’s article. It is the current flagship of the X-series, and it features a 1DD + 3BA driver configuration with a 4-way crossover. The X4 uses one BA driver for ultra-highs, one BA driver for highs, one BA driver for mid/low, and one 6mm dynamic driver for lows. Also, upon inspecting the unit up close, I see that X4 utilizes a tubeless high-frequency driver as well.

The build time is around five days after Bellos gets your ear impressions, which is blazing fast for CIEMs. All monitors from the X-series come with a 2-year warranty that covers both the cable and the CIEMs. You can find the specs of the X4 below.


– 3 Balanced Armature drivers

– 1 Dynamic Driver

– Frequency Response: 20Hz – 20kHz

– Impedance: 16Ω@1kHz

– Sensitivity: 116 dB/mW

– Crossover: Integrated 4-Way Crossover

– Isolation: -17dB (internal ATOM)

Packaging and Accessories

The Bellos Audio X4 CIEMs come in a white cardboard box decorated with the company’s branding and detailed artwork of the X4 on the front sleeve. While the packaging is simple, it’s designed to offer solid protection during transportation. Inside, an aluminum case finished in matte black awaits.

The case, which feels heavy and almost bulletproof, displays the Bellos’ butterfly-resembling logo on its top cover. Adding a personal touch, Bellos engraves the owner’s name on each aluminum storage case. Nestled within this case are the 8-braid silver-plated copper cable, two pairs of hypo-allergenic latex Air Tips, an elastic cable tie, and a cleaning tool. The tips, besides being comfortable, feature a unique bore filter to prevent wax from entering the CIEMs. All the accessories are of good quality, with the cable being especially impressive for its skin-friendly ear guide material and pliable structure.

Design & Comfort

The ordering process for the X4 is straightforward. After making a purchase, customers are required to send their ear impressions to Bellos Audio. Once received, the CIEMs are crafted and can be ready in as little as five days.

While Bellos doesn’t offer customers vast customization options, they offer enough to make the product feel personal. The case and faceplate come in three color choices: matte black, gray, and green. The cable is available in black and transparent, and customers can choose between a 54-inch or 64-inch length. Note that Bellos doesn’t charge extra for any of these options.

The X4’s design is a blend of modularity, practicality, and industrial aesthetics. The 3D-printed matte shells, the round section inside the shell bearing your initials, and the chrome Bellos logos on the faceplate all contribute to a distinctive look. The three screws on the faceplates not only add to the design but also provide easy access to the modular driver system for easy repairs.

For my review, Bellos accommodated my request for a 2-pin connector, allowing me to test the earphones with various 2-pin cables I had on hand.

The nozzle of the X4 is a standout design feature. Despite its oval and relatively short design, it’s compatible with a variety of aftermarket tips. Hifiman tips emerged as my favorites, but I also experimented with tips from brands like Edifier, BlockEar+, and Ortofon. The nozzle’s design is unique, housing a high-frequency balanced armature driver at its end without a sound bore.

The entire design ensures no pressure build-up in the ears, whether the earphones are being worn for an extended period of time or when repeatedly put on and off. The X4 can be worn and removed as simply as Airpods.

As for the fit and comfort, the X4 is exemplary. Crafted using individual ear impressions, they fit perfectly. While they don’t isolate as much as traditional CIEMs, they surpass universal IEMs in passive isolation. The design avoids the sensation of the IEMs delving deep into the ear canals, allowing users to converse or even eat without discomfort.

If removal is necessary, the X4 can be taken out and reinserted effortlessly without any vacuum effect. My extended use of the X4, spanning over two months of daily music listening, gaming, and working, has been devoid of any discomfort, proving Bellos Audio’s design work as intended.


The review continues on the next page. Click here or use the jump below.

Page 1: Bellos Audio, The X-Series, Bellos X4, Packaging & Accessories, Design & Comfort

Page 2: Sound, Low, Mid, High, Technical Capability, Comparisons, Last Words


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