I picked up the M-Audio Sonica Theatre, you might want to check that out. It lets to adjust each speaker lever, you can have up to 8 speakers (7.1) and it has crossover adjust points for each speaker pair. The best part, its USB, so I can run the USB to the amp, and use short RCAs from the amp to the USB soundcard (mini-jacks on that end).
I've only listened to it on headphones (waiting final install), but it will mix 2 channels into whatever using circle surround.
It was about $80 or so at newegg.