we've been discussing this in the 1/3 octive thread (about 6 pages into it i think) seems like a great way to go if you get an external x-over. i have been in touch with a guy who is building some with 5+ vrms outputs (for lest then a 'high end' soundblaster card) the total cost of the system would be more because of the added expense of the x-over but i think it's a avenue worth going down. if i didnt want a completely balanced signal source i would defiantely go down that route.
another option, which is what i just removed from my car is the alpine pxa-h700/701. it takes spdif inputs, has graffic eq, para eq, x-over and volume control as well as a bunch of other processing and gives you 8 preouts at 4vrms a piece.