McIntosh MC 2106: HUM!!!

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.

I think it has to do now, with a sensitive combination of 102 db Cornwall’s and poor Source-changing noise of the Eversolo A8. The heavy hum sound has disappeared with the introduction of my Schitt Freya preamp in the loop, and volume control is still performed by the A8… I can live with the small sound now emitting when changing sources from… let’s say ARC to BLUETOOTH to INTERNET RADIO. I never had this dilemma with my Bluesound Node X.

I am thankful that there is now no repairs needed on my McIntosh MC 2105, thus no hum! Amazing.