if there is a better soundcard(USB amp) out there better than sennheiser GSX 1000? simple answer is A LOT.
i totally agree that good stereo > virtualisation.
But Sound is about personal preference, whatever i says good doesnt automatically apply to the others. Better try it your self, go to the really big audio store where you can try an AMP or soundcard, and bring your Sennheiser hd 598 and see which one you like.