Yes, I do have some news. I took the cover off the top, discovered it was 12 gauge steel, seriously heavy stuff. A total of 12 non symmetrical screws held it in place, with a dab of some sort of Locktite like substance. There was one screw in the middle of the cover, which is something you rarely see in high end equipment. I suspect the Marantz engineers knew there was a resonance problem of some sort.
It is not magnetic. A large 1+KVA torridal transformer is the power unit, which traditionally has well behaved external fields. I took a large powerful magnet and ran it over the sides and top of the until while playing bass heavy music that got the resonances rolling. But I could feel nothing from the magnet, which leads me to believe the resonance is mechanical and not magnetic. Even over the transformer I felt nothing.
As for the chassis resonance and sound quality, it is marginally better with the cover off than on, but not to any real significance. The resonance is worst on the sides, top, and back than on the front, probably because of the 1/8 inch thick faceplate.
I'll have to completely disassemble the unit to apply Peel and Stick material, but I also suspect this is treating the symptom, not the real problem. I couple of layers under the torridal transformer is a good idea, to be sure. But, I still remain clueless on the source of the problem.
