If you really want to go the software route, you should check out this page which details adding software crossovers to a setup via the KXProject drivers: http://www.mp3car.com/wiki/index.php...dio_Processing
However, I just bought a 2-way active electronic crossover and installed it next to my 4 channel amp a couple of days ago. So far, I'm very happy with the extra sound shaping that it has given me. The one I bought wasn't very expensive at all..I think I picked it up for about $30.00 new. I have two amps in my car, with the high/mids running through both of them (one is a factory amp I didn't want to remove), so the high/mids were overpowering the subwoofer, but with the active crossover in the mix I can dial in the exact levels for each set of speakers and it works fine.