i second the notion of using a car amp. buy a cheap used car amp from craigslist or something, and a used battery. You might have to use a long stake in the ground to ground it (not sure if you can use the negative terminal on the battery), but it should work fine. when youre done, either resell it or you can use an old computer power supply to run the amp (12v) and play with a diy sub or something equally as clever as your current projects. good luck!
on second thought:
what about the remote wire connection on the amp? i forgot about that... is it required?