Those are two entirely different animals you're asking about. The S6 is definitely one of the best sounding fully tube circuits around, but the GS-x mk2 is completely solid state and a lot less "tastefully distorted" (even-order harmonics and whatnot). I wouldn't go so far as to say the S6 is warm or anything like that, but it has an undercurrent of tubey sound.
P.S. I really wanna post more replies, but I wanna save my 1000th post for the "Show us your Head-Fi Station" thread xD...
You're at 999 so show the station