I am currently using a clarion 3 way active crossover. this allows for 2 inputs, and has 6 outputs, 2 high pass, and one low pass, all at 18 db/oct (much better filtering than most low/mid grade amps).
this is one of the better units that i have found for under $200. if you are interested in something above $200, audio control and alpine both make some great audio processors, that should also accept a stereo input and convert it to 5-6 channels.
http://www.sonicelectronix.com/item_...n+MCD-360.html