Hiub- asked my dad about car amps since he plays with 'em a bit, and they use a 12 volt power source. So if your computer's power supply can provide the 12v power without exceeding it's rated limit, then you'll be ok powering the amp from the computer. However, due to the transient nature of sound (lots of quick peaks rather than a constant norm) I would go with a separate power supply if you can.
As an electrical engineer, putting power supplies in series- pretty much a bad idea. Fires, explosions, chaos, cats & dogs living together will result.
An alternative is a home stereo amp. They tend to be bigger, though cheaper. If you have an old receiver/stereo, you can power your speakers through that.
Hope that helps.
B