The SR-X Mk3 Pro is one of the rarest vintage Stax on the market. They use drivers common to Gamma Pro and Alpha Pro Excellent, and the same housing as the more common normal bias SR-X M3.
While it’s actually possible to DIY a Pro bias by transplanting Gamma Pro drivers into a Normal Bias SR-X Mk3 housing, lately, the few original pairs of SR-X Mk3 Pro have inflated. I remember having called the SR-X Mk3 Pro a “poor man’s Stax SR-4070”, given it was much cheaper, but it’s not the case anymore nowadays.
The headphone is supra-aural and very cool looking (its industrial design is lovely in person). It’s unfortunately also pretty uncomfortable, especially because of the painful headband.

Sonic character

The Stax SR-X Mk3 Pro is a monitor (just like the 4070), having a very linear frequency response, extremely powerful deep bass, a bit of resonance in the upper midrange. The midbass can sound a bit lacking with the wrong source.

When I initially received this headphones, I had only cheap digital sounding sources laying around: a M-Audio 2496 soundcard and AMB Gamma 1.
Compared to the SR-003, the SR-X Pro is one of the few “sub-Omega” Stax headphones that manages to trump it: it’s much more linear, there is no midbass hump, it has more extension in the treble, and the deep bass is simply thunderous.
The headphone is extremely revealing. Vocals are crystal clear. Treble is much more present than on the SR-003, but never sibilant. The whole region between 6 kHz and 10 kHz is completely flat. When the recording is harsh it can be heard so, but the audible effect is more related to thinness (on classic rock from the ’60s), and not related to sibilance nor metallicness.
While the SR-003 does adds midbass to the sound and lacks some lower treble (or, without the headband, manages to sound light in the deeper bass regions, and in any case smaller sounding), the SR-X MkIII Pro doesn’t do almost any colorations, behaving like a magnifying glass for good recordings.
The only deviation from a flat response is a certain aggressivity in 3 kHz reguib, that can make female vocals sweet, but sometimes screechy when the singer shouts loud. In general, paired with the good source, it manages to sound very aggressive and “in your face”, as some like to say.
They might not be perfect for ’70s rock, since the bad recordings come out the way they are… and flatness for rock is not always involving.
Recordings with deep bass are lovely, and an example is Peter Gabriel’s “Passion”, which becomes spectacular and feels like playing in a huge surround cinema… just shrinked in a very small soundstage around the head, since the SR-X Mk3 Pro, while sounding bigger than the in-ear SR-003, is still a narrow-sounding headphone, compared to Omega 2 and Lambdas.
When the recording allows it, it’s possible to ear the sound at different layers of detail. The more you pay attention to it, the more nuances you’ll hear, like if looking at the sand closer and closer.

I had the chance to compare the headphone on Stax SRM-1 Mk2 Pro and the ‘tiny’ SRM-252A… while I believe the SR-X Mk3 Pro isn’t exaggerately power demanding, and something like the SRM-1 Mk2 Pro or equivalents (SRM-T1S, SRM-323) are more than enough to drive them effortlessly, the small, low power SRM-252 makes them sound a bit uncontrolled in the bass region (slower transients) and causes a loss of resolution in the upper treble region.