check the faq forum and it'll show you how to connect a pc w/o using a head unit.
as far as controlling the volume, you can do that on the software side and then go out from your soundcard into:
a)(if your soundcard has multiple outputs) run 1/8 min to rca cables from your soundcard to your amps
b) active crossover-- (i.e. audiocontrol 24xs) go stereo out from the soundcard or usb dac into the active x-over and that'll split up your signals to go to your amps inputs
c)stereo out from dac or soundcard into an amp with passive outputs (i.e. elemental designs' amps have this)-- you'd hook it up like this: soundcard-> main amp input, main amp passive output-> sub amp
if you're looking for a really nice budget system I'd suggest going with a soundblaster live 5.1 pci card (install the kxproject drivers) and then go into a audiocontrol 24xs x-over/linedriver.
i had that setup in v1.2 or my install and it sounded suprisingly great.
do yourself a favor and go read the faq emporium before you ask another question. there is a ton of knowledge w/in those pages