I'll second that theory. Was just checking into sound cards not long ago and was amazed at how many of them, higher end included, lack onboard amp IC's. What a trip.
MAKE SURE you bring your levels in the main master control panel back down to 3/4 or less and the master it's self way down before you go hooking up a powered speaker setup!!
Another side note, I just got the Labtec Pulse-424 set, and for $50 this set is clean and thumpin. I have to keep the sub almost all the way to lowest level as it's TOO MUCH bass. And not just mushy WHOOVE-PHWOOBE crap, tight, clean, and punchy bass.
Yeah, yeah, it's not the 5.1 digital thing for those of you thinking "what rock did this guy crawl out from under" but judging from the soundcard mentioned you're not looking to go all out HI-FI or spend lot's of money, yes?
Found Labtec's at Office Depot, and Sotware Etc. so far.
Good Luck-
Bomber