There is a big thread going on right now about using the PC for EQ, time alignment, x-overs and using outboard DACs to run the signal to the amps. No external processors needed. Not very expensive either, computer cost=?, DAC cost=$140 per 2 channels. A 2 way setup would need 2 DACs, a 3 way, 3 DACs. This setup should be super high end, better than all the processors in this thread.
http://www.mp3car.com/vbulletin/showthread.php?t=96332
Good stuff is at the end.