Ampeg Gemini VI rebuilt, now has suspected ground loop

[Big Dave]

Hey all. I inherited a completely destroyed Gemini VI recently. Replaced all filter caps and all coupling caps(some were leaky so I replaced 'em all.) Replaced diodes and tubes(most were bad, converted 7199 to 6u8a-type to find a home for some groovy Amperex 6LN8's that I have laying around.) Got it biased up and it sounds great. Replaced the missing speaker with a 1970 Utah ceramic mag and it really sings. The amp was pretty trashed when I got it, and I think that I failed to follow the ground scheme correctly. There is a loud hum that gets very loud with the treble turned up(bright switch makes it extremely bad) and a feedback loop in the reverb effect. When the treble is turned up and the reverb is turned up there is a noticeable high frequency resonance from the reverb tank. This can escalate into a full-on feedback loop, even with the tank isolated from the cab on the workbench and insulated in a home made reverb bag with open-cell foam insulation. I know the grounding scheme on the old Gemini's was pretty particular, and I doubt that I re-created it properly. Oh yeah, I replaced the blown power transformer with a unit from a Magnavox stereo console(It powered a quad of 6V6's and a pair of 5y3's, plenty big for this 7591A push-pull.) Methinks a ground loop is the culprit, and I am hoping one of the old Ampeg experts on this site might have a detailed grounding diagram for the Gemini VI model. Even a detailed written explanation of grounding scheme would be very helpful.

Thanks!