When mine isn't hooked up to Vinyl (no sibilance at all) I have a nice cheap little u-DAC2.
It seems like a really excellent match, actually... the K272 and the Grados I've had don't pair nearly as well as the W1000X does with it. It's really quite textured and rich; possibly because both are slightly "romantic" (read: boosted lower midrange slightly and warm).
It's a cheap solution in comparison to other things out there, sure. But it bears no hiss, and really comes out sounding absolutely incredible with excellent imaging. Works well for games too. Having heard your soundcard I can say without question that it IS the culprit. Creative stuff is for the lack of a better word... garbage when it comes to audio. I've heard that they've got some crazy new card out there that is "suitable for audiophiles" but I've heard that from them before.
I've listened to the W1000X out of a few things now, and I don't think they're particularly sibilant. I think that they pick up on sibilance in other gear very easily, but only if it's already there.
Edited by Mercuttio - 4/24/11 at 11:54pm