[DaveWM]

try replacing one of the .01 grid coupling caps for the difference amps, see if that changes the voltage reading on the grid of the 6GU7 diff amp tubes.
there are 3 of course, but you may as well only do 1 (say the blue) and see if there is a change. check to voltage before and after the change of the cap.
think you said you were getting around +2v there, I show -2v should be there with no signal.

if you can reach it, do a quick test, check each side of the 6Gu7 .01 diff amp grid coupling cap to ground. report back voltages

on is behind one of the 6Gu7's the others are near the front of the board.

Normally I would just test for leakage, but since you do not have the correct tester you need to replace or rig something up.