hmm, i had that same exact problem w/ the bass hitting w/o any music, i would play the music, and they would start hitting and i would turn the volume all the way down and the subs wouldnt stop. that was shortly after my alternator whine problem.
to fix all this i went the expensive route, by getting my amp serviced, and getting a new HU. i also sanded the hell outta the metal where i had my amp grounded and checked all wiring making sure they wern't coming in contact w/ something they shouldnt.
so check your amp ground and power wires. if that don't work try hooking up a friends HU to your system and see if that works.