It is not DC issue. When there is no signal, all legs are at perfect 0 DC. It is when the signals begin to swing, one lead of the amp output will swing up and another will swing down, referenced to ground. To demonstrate how hazardous anyone try to connect BTL to plate amp via high level inputs, I have a customer did just that and the entire preamp board is fried. This is because the negative leads from BTL amps output are not at the same voltage level (unless your left channel is exactly same as the right channel). Now if one says but I have two subs, one for each channel, would that work? The fact that the negative lead is connected to the speaker level input black terminal, meaning ideally, as soon as you start playing, the same disaster should have happened. However, some have reported successfully working this way ONLY BECAUSE the amp does not have power ground pin (ie it uses 2 prong power cable). If this gent accidentally touch the subwoofer amplifier chassis when it is playing, he can get electrical shock because negative speaker level input is essentially tied to chassis ground. REL speaker level inputs are like XLR inputs. In that case it needs 3 wires. No two-wire system can work without creating some sort of hazard.
Think of it this way, in any signal transmission, at least one wire needs to be ground. Without that power ground takes over signal ground. When there is neither signal ground nor power ground connection, the chassis becomes floating.