Want to go with a car battery and a car battery charger? No hum problems.
(You must turn off the charger before turning on the amp since the amp will always impose its load on the charger first because all chargers feed a voltage higher than that of the battery, also the charger may introduce hum.)
Very inexpensive.
Video hints:
http://members.aol.com/ajaynejr/video.htm
By the way what is the THD (total harmonic distortion) rating on your subwoofer amp? Although I have not researched car audio recently, many years ago car audio on average had higher distortion figures compared with home stereo.