Fender M80 : distortion

[buck1107]

Interestingly .......... and despite all other variations on a theme ..........
from memory, the problem was the pre/amp in out jack socket/s. A bit of spray in there and all was well.
From memory, tho ......

Glad you got it working! Yes, those effects send/return (pre/post amp) jacks can present some interesting symptoms!
Happy playing!