McIntosh MC 2106: HUM!!!

Hi Moose - looks like you answered your own question.  One of the (however many) components you disconnected/connected is either not connected properly,  is using a cable that has issues, or is plugged into a different AC circuit than it was in previously.  Start with the basics- amplifier and speakers with nothing else connected.  If no hum, then connect the pre/pro. - If humming- there is your problem.  If not, then connect first source and keep repeating until you find the culprit.   Good Luck!

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.