Hmm, guess I'll stick with the HA400 then. A friend actually already bought the setup you suggested, same headset & Sound-card and said the difference wasn't that great; until at higher volumes where it didn't loose quality or gain noise.
Still, I guess, what can you do? I know it's a headset, but dammit that doesn't mean I can't have good audio too.
http://www.head-fi.org/forums/f4/modding-sennheiser-pc350-headset-406187/
Thinking about doing this also; worried about destroying Headset. Thing is, 9 pages + of people who did it and even an Official Sennheiser Rep endorsing it...I just don't know. Taking a drill to it seems risky, I don't have any small drills for it...though I did read someone using a Thumb tack, I may try using one of those. 2-3 holes is the sweet spot I'm told, to get HD595 quality.