llamaluv - I see you own both the HEK v2 and Utopia headphones. I would be very interested to hear from you a comparison between the Studio 6 and GSX Mk 2 on both headphones.
Thanks for asking. I've been meaning to write something about my Studio Six experience, so...
HEKv2
I think the Studio Six drives the HE-1000v2 quite well. Fast, resolving, controlled, way more power than needed for raw SPL. Tonality is maybe just a shade warmer than neutral. Bass run deep (though no tube amp can compete with the depth and detail of the lower bass on the GSX, is my theory). No pronounced tubey midrange bloom.
The GS-X's renowned transparency makes it hard to compare without taking consideration of the DAC that's feeding it. I have it connected to a Pavane, which gives it a full, "analog-like" sound with a suprisingly expansive soundstage (qualities usually associated with tubes, basically).
With that in mind, I hear the two to be not far off in overall character. I do prefer the HEKv2 with the GS-X mk2 though. Soundstage is a little wider and rolls off more gradually. And the almost aggressive character to the GS-X's treatment of dynamics suites the HEK better by partially offsetting its soft-ish sound presentation. Tube rolling on the S6 didn't have much effect on my impressions. Also, I think because I personally find the HEK very full-sounding already (maybe due to the slow decay that shows up in waterfall graphs?), any tube-effected "harmonic magic" becomes less of a potential plus-factor than it would otherwise. That's a theory, anyway.
I actually also prefer the HEK on the Kenzie Encore over the S6 as well. It has a nice tube-effected, opened-up midrange, and most of all, the soundstage is big and very satisfying.
Utopia
I like the Utopia on the S6 the best. On the GS-X, it's sometimes too aggressive for me, and while super-resolving, oftentimes doesn't hold my interest after the first few minutes for whatever reason. And while better connected to the Pavane than on previous DACs I've tried, the upper frequencies make for a "challenging" listen that I'm not always up for. And on the Kenzie Encore, the 32-ohm output impedance on my particular model gives it a little too much bass bloom.
On the S6, it's nice and lively, articulate, has a richer tone, and is overall more "musical" to my ears (which is a hand-wavey way of saying that it "does it" for me in a way I don't quite know how to articulate, lol). I wish I had more to say on it, but I've not been giving the Utopia much time for quite a while now, as I've been gravitating more and more towards all the planars. I also really liked the ZMF Eikon with the Studio Six for similar reasons.
I love the Studio 6 - I just wish it had a bit more power for the LCD4’s and Abyss Phi’s of this world which are the next step on my journey...
Having recently owned the LCD-4 and just gotten the Phi, I have to agree.
I've put my Studio Six is up for sale and am going to go for a high-powered tube amp in a different price category next.