ATI AT6007 amplifier hum

I had a hum with my ATI Class D amplifier that I use for HT. It was a ground issue that was corrected by simply running a inexpensive shielded wire from the ground on the amp to the ground on the Marantz preamp/processor. 

If you unplug everything else, do you get the hum on the ATI?  Have you tried moving the ATI to another room and testing for hum?

I have almost your exact system; a 7.1.4 using an ATI6005 for my fronts and sides, and an AT527 class D for my rears and atmos. I also have three dedicated 20 amp lines. I have never had any hum issues. In a previous system I had a hum that I traced to a cable box.

Do you have hum just coming from the amp or do you hear it in the speakers?  I have 2 lexicon RX-7s which I think are similar to the ATI. One hums a bit but it’s in a rack in a closet and speakers are dead silent. Like you they are on dedicated 20 amp lines with shunyata viper cables. I have never chased the hum but now I’m intrigued if something in the chain is causing it.  I did have DC offset coming through two Fathom f212v2 subs from my plasma and tivo mini.  I got a $15 screw on for th tivo mini which took it down 50% Andy then still needed an emotiva CMX-2 for the subs to take it all out. That is 15 amp though.  I do have two furman P 8 Pro C units for sale on Pittsburgh craigslist and would be happy to post and sell them on here if you have any interest if that could help you out.  They are one of the few 20 amp items I could locate that weren’t $$$$. 

This Isotek unit works on 20A.

https://www.musicdirect.com/power/IsoTek-EVO3-Syncro-Uni-Sine-Wave-Rebalancing-Unit-C19