OP
After looking at your image, if you play any audio from Rack 1 out of the Sonance Amps located in Rack 1, do you get the hum?
If the answer is no, then the interconnects that are being used to connect from Rack 1 to Rack 2 are introducing the Ground loop to Rack 2.
Back to my original post, most likely your power outlets are on different phases.
EDIT: you could also try a powered audio balun, although that may not fix the original problem.
Last edited by lippavisual on September 8, 2017 17:15.