you might want to look through the sticky in the car audio section about amps with optical inputs. I figure if you want the best sound reproduction from a digital source, your best bet is to keep it digital all the way out to the amps. This can be a pricey way to go no doubt, but if you're running TOSlink or some other optical format you'll be immune to noise and loss through those lines.
As far as sound out from the computer goes... digital is digital. frequency response out is exactly what you put in, so all you need to do is find a sound card that has the optical output to go to the amps. As far as adjusting the sound, I think you should be looking at whether or not a software based EQ would suit you better than a hardware EQ or vice versa.
For the best sound quality you might investigate what recording studios use... so places like guitar center or musician's friend (if they're still around) would have some leads for high end computer audio equipment.
Just my thoughts.