Actually managed to fix it in the end somehow by just opening up the case and tapping the connections for the center channel. I think it was a solder point that was loose, but I decided upon opening it that it would be a significant undertaking to re-solder loose connections. Ironically the tapping seemed to resolve the issue. Prior to that, it was reading equal voltage and resistance out from the center channel as the side channels. I believe it was just a connection signal output issue, as the power was there, just not the clarity and expected volume.