My understanding of common ground is that both the right and left channel have a common ground connection (I could be wrong but if so I am also sure someone will correct me ). Since the M-200 are monoblocks they are not a common ground. You can tie the grounds together by running a cable between the ground connectors on each amp. I do this with my SDA-1C's since they require a common ground when the SDA cable is attached between speakers. Then you could run a third wire to your REL from the right amp.