I’ve tried 4 configurations and each results in loud hum from the sub - even with the main system powered off. (1) Floating (leaving unconnected) the black ground wire from the REL wiring bundle. This is the solution the REL manual says will work. (2) Connecting the black wire to an unused RCA connector on the preamp. (3) Connecting the black wire to the chassis ground terminal on the S300. (4) Connecting a cable between unused RCA connectors on sub (specifically, “LOW .1/LFE” as well as “LOW LEVEL INPUT”) and unused RCA connectors on preamp.
I had been using the REL without problem with a previous amp, so I know it works fine. Seems like it is incompatibility between the REL and the S300. Unless someone can tell me that they have had success, and how, I think I’m giving up on the S300.