This may sound really dumb, and I don't have even a smidgen of the electrical chops that the old hands on the forum have; but I was having a similar problem awhile back with some new equipment and decided to make my own DIY "ground boxes" that use the RCA connections as a "signal ground." I found that in my system signal grounding only the CD transport and the DAC plus the REL subwoofers worked best and that every component need its own dedicated box. My boxes contained mostly sand and powdered graphite - the kind that hardware stores sell for lubricating door locks and such. Nestled in this mixture was a copper mesh conductor linked through the box to conductive wire leads soldered to RCA male connectors which interface with the female RCA connectors on the back of the components.
The upshot was that any little hums vanished and that the music became more relaxed sounding.