The amp in my 120 was potted in wax when I got it. All along the base of it and around the edges. Had to chip it all out to even get the amp out! Even after cleaning, the amp has a nice waxy residue to it - ick. Was that something original to the piano? I'm asking because I'm trying to chase down some hum, and because there's still some wax that remains on the case, I don't think the amp chassis makes perfect contact with the electrostatic paint, which I would think it would need to.
First of all, that sounds hella weird. I've been rebuilding the amp to my 120, and haven't heard of anything like that in my research.
Second of all, I'd recommend switching out the two-prong plug for a three prong so you can ground the amp chassis to your own ground. There's always going to be hum, but there are some great electrical guides specific to the 120 at tropicalfishvintage.com that can reduce the hum enormously.
And remove the death cap!!