Well, I got to the bottom of it. The problem for the 4-channel amp was that the fuseholder was burned out inside, making for an inconsistent connection there. Taking it out, bending the teeth and popping it back in seemed to do the trick for that amp. But once I did that, after turning the car & stereo off, my sub amp was staying powered on and I couldn't figure out why. Turns out that about two or three frayed strands of wire from the 4-channel's power wire were touching the remote lead terminal on the amp. This, in turn, was a relay that was sending ~8V power to the remote lead of the sub amp (since the remote wire was spliced into two), keeping it on and draining my battery. I wonder how long that'd been taking place. Thank you, voltmeter.

Thanks dleccord, wrxkiller and orbit for your help...everything seems to be straightened out now h: