I could use some help with a "mystery" humm

I’m hoping to get some ideas on eliminating a hum in my system. It is an unusual situation, so I will try to be as specific as possible, without being to long winded

I have recently been replacing all of my single ended IC’s with XLR’s. My last component to switch over to balanced was my McIntosh MEN 220 room correction unit, to ARC pre-amp. I powered up, after switching over, and my previously quite rig now has a nasty hum, that is unacceptable. I tried various different XLR’s, to no avail. Once I switch back to RCA’s between these two components, all is good once again. I spoke with McIntosh, and some earlier models of the MEN had similar issues, but the problem has been rectified with the later models. I was asked to bring it to my local McIntosh service depot for inspection/testing. All checked out fine. The tech suggested I try a ground lift on the power line out of the MEN. I did this and it worked!! I proceeded to take every power cable out and check each one out for correct phase, as McIntosh told me that they have seen power cables miss wired quite often.

I am running 2 dedicated 20 amp circuits, each one delivering to separate Furman Elite 20PFi conditioners. I won’t bother listing all other components, as I don’t think it matters, but if anyone thinks it might, you can check on "My System" Bye the way, I have no television in my house, and my room is a dedicated listening room, so no other non audio electrical devices are present there. If anybody can suggest options, I’m all ears. I do not like the idea of keeping the ground lift in place. I should also mention that the amplitude of the hum remains constant, regardless of where the volume control is set, and the hum is gone when the MEN is removed completely. from the chain

Thanks