I previously discussed my experiences with the Rane PE-15 to reduce room resonances for my sub. I had some intermittent hum at first that went away with some fiddling. Well, I was on vacation for a week, actually unplugged everything from the wall, came back, powered up and SH*T there's that loud 60Hz hum coming from my sub! I fiddled with the connectors again, i.e. turning them and pulling out and putting back in repeatedly, and no improvement.
But, before I went and tore apart anything or built better cables, I sprayed some contact cleaner into the 1/4" connectors in the PE-15 and the noise is gone and the contact appears solid.
So in my case, it wasn't a ground loop hum but the RCA cable connected to the sub amp having one side floating (i.e. acting as an antenna instead of the opposite polarity of the other side).
Lesson 1: Contact cleaner is your friend.
Lesson 2: 1/4" connectors stink because you can't see the contact surfaces.