I should get the chance to hear the GS-X at the Charlotte meet next month since HeadAmp is a vendor there. Will be curious to see how it compares to my Questyle with my HD800. Someone in the other thread said they like the Questyle over the GS-X. Honestly that wouldn't surprise me since they built the CMA800R with one purpose in mind, to sound amazing with the HD800. Then again, I think that was the same purpose Sennheiser had for the HDVA600/HDVD800, and it's reception has been lukewarm.
If your talking about this amp: http://www.amazon.com/Questyle-CMA800R-Current-Headphone-Amplifier/dp/B00GM7F8E8 I got an od chance to hear it a week or so ago. On a pure humble as I never really knew anything about the amp.
In short. It rocks with the HE-500s / not so much with the HD800s IMO. Although it did sound pretty good, It lacks the transparency and the full bottom end of the GS-X mk2. I still have not heard any of the Sennheiser amps so I can't make any comments.
The GS-X mk2 is still the best SS amp I've heard with the HD800.
In order of preference and IMO..
GS-X mk2 / F1J
4 Channel Beta 22
I would rank that CMA800R somewhere with my M^3