Have you considered using the FAS "Humbuster" cable design? You can make one yourself and give it a shot in an attempt to eliminate your ground loop. I've made and used a number of these in the past when a ground lift or hum wouldn't go away, here's a link to read up on it a bit more...
I have an 800 from '86 that I've always considered putting on the market.
This amp has been completely gone through, tuned up, replace all faulty components and various additions incorporated that will get the amp tone and sound of ANY Marshall amp you'd like to venture into.
The "Cork...