i am not using the soundcard you describe, but a mixvibes U46dj and a terratect mkII (line in)
Had the same problem you have, it doesn't sound nice, its flat and no base from the fronts, only sounds good when the sub is on. The onboards just don't solve that problem.
However i also use SRS_Audio_Sandbox and it has to possibility of choosing the size of the speakers in my case 60 Hz. Its not free, but you can try it for a few weeks.
My cpc in the current config pumps out the same sound as my head
You could also have a look here (usb sound cards)