Paralleled batteries will give you increased current capability - at the same voltage.

Jeff Mackwood

ps. While this sounds like an interesting "what if" project, I still don't understand its inherent advantages (if any) over using an amp designed for home use. If the real driver is cost, then I'd suggest picking up some good used amps (like NADs, Carvers, etc.) off eBay - likely for less than the cost of all the bits and pieces you'll need to buy to use car amps - and with better sounding results. I've acquired five different stereo power amps that way.

ps. I remain highly skeptical of the claimed power output ratings (and other specs) for most car amps. I have a hard time equating 1 claimed car amp Watt with 1 claimed home amp Watt.