Hum when MC225s in Mono

I have just connected, new to my collection, just back from my tech MC225s to a new to me MX110 X series. I've decided to use them in mono with Hereseys. Following the directions from the owner's manual, I have set the switch to mono and strapped the commons together and the 16ohm outputs together for the 8ohm speakers. I have the gain set to the center position on the right channels and the lefts all the way counter,clockwise. I have both amps connected to the switched outlets of the MX110.

When I power the system on, I believe I hear a hum both in the amps and being outputs from the speakers. My music source is a Mac Mini playing through DAC connecter by USB.

Any suggestions for trouble shooting, isolating or otherwise eliminating the slight hum?
 
Have you disconnected the input source to the MC-225's? If so, and the hum goes away, then the issue is probably the MX-110. Let us know what you find.

Mark
McIntosh Factory Center
 
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