Curious what MLundy finds. I've run separate hard wired feed from main box to the audio room only. One single cryoed (plugged both grounded and ungrounded) line to a UberBuss feeding all components, to no avail. Two of Danny's filters lay waiting. I can insert one before a monoblock with no noise, insert the second, and hum returns. I'm beginning to think as most studios apparently, an entirely separate outside ground rod is suggested.