So just down a wire, you can repeat along the line, and then one power source. (the music signal) should be able to "phase cancel" all the power as long as its spread out along the line.
Its not just subtraction doing it, it actually involves the two power sources (the music, plus the power from the supply.) actually going in series and diverting the power in a slightly more complicated fashion, it has to be doing that because it inverts the power on both the right going and left going current, so its not just adding on, its a little more complicated activity involving the two power supplies in series.
I need to go to the shop and get some speakers, then just need cheap 100 ohm resistors (1/4 watt will do, because its all spread out only 2 volts per resistor), and it operates a little quiet for 2 volts on each speaker because i need to add the load for it to work properly, but it should work taking that into account.
Ive tried to collect all the amps from the power lines onto one line, but alas I couldnt do it, it seems to go into series when u do that, and it goes double volts but double the resistance, so its the same amps. (it wouldnt do it with this one... cause its all powered from one source --- go double volts double resistance, but the multi battery version does.)