I am using same setup..4 channel amp for speaker and 2 channel for subs.
I use an OLD Sound Blaster Live 5.1. About 20 bucks on ebay. Use the KX drivers with it and you have an amazing sound stage. I run from the sound card, Rear out. Front out, and Sub/Center out to Rockford fosgate 3sixty. This helps because I can change the front and rear sub levels independently. If controlling the subs separately is not a concern of yours than they go right to the amps! You have to use 3.5 din (Standard Headphone Jack) to Split RCA (red/White).. Get them at radioshack for 5 bucks. You can get higher end cards but I find the KX drivers give you more capability and sound quality for Car audio than anything else.