Hmmm.... I did suggest disconnecting the coupling capacitor
not the grid resistor (leak or stopper). It does sound like a plausible diagnosis has resulted though
The 4k7 resistor on the grid will be the stopper and a drift in its value per se to 6k won't have much effect but it wouldn't hurt to replace it
and the 470k grid leak. Use a metal film type or at least a good quality carbon film job. Dropping the grid leak to 330k would also help a bit with the tired UL84 at the expense of a slight volume loss.