Thanks for your replies.
Phil A. None of the REL videos or posts address this issue precisely enough. I've reviewed them.
Sunny daze. There is some debate about (a) -- if it's grounded on the chassis to avoid hum.
Re: (b) Good point about the variable. I need to check to see how it was wired, exactly. Just to review:
My question: "So, before I ordered the preamp, I had Mike wire the "Tape Out" as a *second* main out. Right now, the preamp's MAIN OUT goes to each monoblock and the other (TAPE OUT -- now a second main out) is free. An alternative is that I get a Y cable that will bring two RCA outputs from my Preamp into the single RCA input on the REL?"
Answer I received: "Don’t do that! It would result in the main speakers reproducing mono. And although I doubt that it would happen in this particular case, with some preamps doing that might even result in damage, eventually if not sooner, since you would be shorting the preamp’s outputs for the two channels together. Presumably the change that was made disconnected the two tape out connectors from the internal circuit points to which they are ordinarily connected, and connected them instead to the main out connectors (or to corresponding internal circuit points). So if you were to connect a y-adapter to the two tape out connectors, to create a mono signal for the sub by shorting the signals on the two tape out connectors together, you would also be shorting the signals on the main out connectors together. Thus an identical summed-to-mono signal would be provided to both monoblocks.