You need an amp to drive a sub woofer I'm pretty sure.. I've never heard of any application in which you didn't need some sort of amplification.
Also why wouldn't you want to run the power off the battery? I have 2 12" subs running off the stock 3400 electrical system... the Ecotec charging system can easily power a 6.5" sub. Getting a separate power source isn't worth the time or money.
__________________ Hit a dear! Back to stock 3400... For now
why not just repace your deck and your rear speakers at that point, i have a kenwood deck, no amps and kicker 6x9's in the rear and they still bump pretty hard, the factory delco speakers are shit anyway