Contact Sumiko. They have a special cable you can order that has an RCA termination instead of the black speaker wire that uses an unused RCA plug on your preamp for ground (or power amp if you're going balanced). I believe it's $200 if it's worth it to you.
Also, you may want to contact the member Zikhmark, who sells an aftermarket REL Neutrik cable here. Perhaps he can make an RCA ground version for you. It would probably be cheaper as well as an upgrade to the custom speakon from Sumiko/REL!
Also, you may want to contact the member Zikhmark, who sells an aftermarket REL Neutrik cable here. Perhaps he can make an RCA ground version for you. It would probably be cheaper as well as an upgrade to the custom speakon from Sumiko/REL!