I had the hum issue on class D monoblocks. I followed RELs procedure for Class D fully differential amp connection and it hummed like crazy. I then connected using both leads (+ and -) and it’s dead quiet. There’s some risk though as REL says not to do that.
I thought the .1 connection had to come specifically from a receiver with.1 output as it bypasses the internal xo in the sub, so you can’t connect from a pre out on a standard preamp to the.1 input. Maybe I misread that part in one of the posts above.