Ground Loop Problem - Stumped

I've got a nasty ground loop problem that I can't seem to figure out. I've done extensive research and tried just about everything but to no avail. Here is how I have everything connected. I am using the ExactPower EP15A and SP15A. All of my sources both digital and analog are plugged into the SP15A which in turn is plugged into the EP15A. My amplifier is plugged into the EP15A. The EP15A is plugged into my wall outlet.

1. I removed all the interconnects between all the components but kept the speakers connected - NO HUM.

2. Connected my preamp (Meridian G68) to my amp (EAD PowerMaster 8300) via RCA interconnects - HUM.

3. The hum is not affected by volume level.

Any ideas?

Thanks