Did you notice the arc when you disconnected the power to the rear amp, or when you REconnected it? Audio power amps have larger capacitors in them to filter noise. If you left it disconnected for more than a couple of seconds, it would have taken a lot of current to charge them back up and then the current drain would have dropped near zero.

Has somebody actually confirmed that the battery goes dead by checking terminal voltage, or is everyone ASSUMING it's dead by symptoms? A loose cable at the starter or some missing/intermittant grounds will cause similar symptoms. You could be hunting down a non-problem.

For the fun of it, unscrew the voltage regulator from the alternator and see if the brushes are at the end of their life - and double check the charging indicator on the dash (key on, engine stopped = light on).