McIntosh MC 2106: HUM!!!

I called McIntosh tech. They suspect a transformer because the L gain hum increases with additional volume while the R channel hum volume stays the same. (Testing without any components). They suggest finding some to test it.

You might try one of the other speaker outputs. That will use different windings on the autoformer transformer.

Try lifting the ground at the preamp power cable.
Ground only the component which draws the most AC current and lift it for all the other components.

Ground passing through line and cables at the same time can induce ground loops. 

https://silversolids.com/

@Stager

Agreed, I had a groundloop in my system, eliminated it by using the adapter 3 - 2 prong and amazingly all the buzz went away. It seemed like magic, but it's actually physics.

Hi Guys.  It's great that lifting the ground stopped the hum, but you have not fixed the cause of the hum.  Using a cheater plug is a temporary measure - the ground is there for your safety!  FIX the cause of the ground loop and lose the cheater plugs.