Wow, excellent explanation of the situation (and general audio/electrical physics too). I just had a minor question as an aside. I may purchase an amp capable of 10V line-level inputs (got some Kickers in mind) to power a new subwoofer. After I have located the signal wires from the radio headunit going into the factory amplifier, what is the best way to tap them so that the signal is split to both amps? The balanced 5V seems like it would need a special wire harness or something to maintain that +/- charge separation.

Thanks in advance for the help!