You say that for the left speaker, 2 connections out of 4 on an XLR type connector are not used, but you don't make it clear if the sub also has a 4 pin connector, too.
I mention this, as it is only necessary to have 2 connections for normal passive speaker units, so I have no idea what the other connections are for, or even if they are used, but it is just possible that they carry some power-supply DC voltage (for an internal amp) from elswhere in the system. that there is no other power supply going to the sub, and there is no amplification in the sub, either. My comments only apply if this sub is all passive, i.e. Regrettably I have no experience whatsoever of this sub, but I do know how speakers need to be connected-up to operate.
