My problem isn't with my rear speakers, though. It's with the subs that are already on the same circuit as everything except the rear amp.
So not sure how common mode rejection is in play.
And yes, I could just run the circuit to the back... but that's not the solution I'm really hoping for. The ground lifting XLR plugs sound good, I just am confused by them.
Here is what is happening:
You have an AVR or AVP connected to mains (presumably at front of room). This is connected to your subs via unbalanced RCA. Your subs are also connected to mains, and grounded there.
You have (long) xlr across the room (through roof space??) to the rear speaker amp at the back of the room. This is also connected to mains at that end of the room.
There is an earth connection between the two mains connections - either around the room (if on the same circuit) or meeting back at the distribution board if on separate circuits.
So you have a ground loop:
Mains to AVR and/Or Sub - Through RCA shield to Sub - to mains (sub mains connection) - through mains earth connection in wall/back to distribution board - back to mains at back of room - to rear speaker amp - through XLR shield and finally back to the AVR.
This is a huge loop. Any stray magnetic field (eg from mains currents) passing through the loop gets picked up (Think of the earth loop as a single turn secondary of a transformer) and generates AC currents around the loop. This AC current passes through the shield of the RCA connection and causes an AC voltage difference across the impdeance of that shield. Since the connection is unbalanced that appears as an AC noise voltage at the input to the sub (added to the signal from the AVR).
Same is also happening on your XLR cable shield - but since that is a balanced connection, the common mode voltage is added to both the hot and cold signals - so it gets cancelled when cold is subtracted from hot at the input to the amp. So the XLR connections are doing what they are supposed to do else you would get noise on the rear speakers as well as the subs. This is the common mode rejection.