Reviewed on: SoundStage! Solo, September 2019

I measured the MX4 Pros using laboratory-grade equipment: a G.R.A.S. Model 43AG ear/cheek simulator/RA0402 ear simulator with KB5000/KB5001 simulated pinnae, and a Clio 10 FW audio analyzer. For isolation measurements, I used a laptop computer running TrueRTA software with an M-Audio MobilePre USB audio interface. The headphones were amplified using a Musical Fidelity V-CAN. Except as noted, all measurements were made using the supplied medium-sized, single-flange silicone eartips, as these fit the ear/cheek simulator best. These are “flat” measurements; no diffuse-field or free-field compensation curve was employed. If you’d like to learn more about what our measurements mean, click here.

Frequency response

The above chart shows the MX4 Pros’ frequency response, which is a fairly “textbook” response except for a boost of about 4dB centered at 4kHz. This looks like a very deliberate voicing decision, and it’s the reason for the lower-treble emphasis noted in the review.

Frequency response

This chart shows how the MX4 Pros’ tonal balance changes when they’re used with a high-impedance (75 ohms) source, such as a cheap laptop or some cheap professional headphone amps, or some exotic tube amps. There’s about 1dB more bass at 20Hz and 1dB more treble above 3kHz; my guess is that using a high-impedance source will make these sound just a tad brighter overall. Note that this is much less variance than I normally see with earphones using balanced armatures.

Frequency response

This chart shows the MX4 Pros’ right-channel response compared with the Campfire Comet (single balanced armature), 1More Quad Driver (one dynamic driver with three balanced armatures), and the AKG N5005 (one dynamic driver with four balanced armatures; when used with their reference filter, these earphones are said to best conform to the so-called “Harman curve,” the response that research shows delivers what most listeners consider the most natural sound) earphones. Clearly, the MX4 Pros’ deviation from the norm is that big 4kHz peak.


The MX4 Pros’ spectral decay (waterfall) chart looks mostly clean, except for some well-damped resonances at about 3.2 and 11kHz.


The total harmonic distortion of the MX4 Pros is fairly mild, not even breaking 2% at the extremely loud listening level of 100dBA. What’s unusual, though, is that the distortion tends to be higher in the midrange than in the bass; this is probably because the balanced armatures can’t match the power handling of the dynamic driver.


In this chart, the external noise level is 85dB SPL, and numbers below that indicate the degree of attenuation of outside sounds. I chose silicone tips for this measurement to ensure a level playing field; some of these models (especially the Campfire Comet earphones) will achieve much better isolation with foam tips. Isolation of the MX4 Pros with the silicone tips is outstanding, probably because of the ear-filling design and the over-ear cable routing.


The impedance magnitude of the MX4 Pros is admirably flat for a hybrid model. It’s about 9 ohms when you’re in the range of the dynamic driver, and once the armatures kick in (apparently around 1kHz), the impedance rises to 19 ohms at 20kHz, measuring about 33 ohms up to 1.5kHz, with a couple of slight impedance peaks that correspond with the frequency response peaks in the treble. Impedance phase is fairly flat, as well.

Sensitivity of the MX4 Pro earphones, measured between 300Hz and 3kHz, using a 1mW signal calculated for 12 ohms rated impedance, is 100.7dB. That’s a little low for earphones, but still plenty enough to ensure loud volumes from almost any source device.

. . . Brent Butterworth