Your issue needs to be repaired before you think about mods. The schematic shows me that the input is not capacitively coupled and there's a bias path through the pot even if the 1M resistor goes bad. The other place to look is to verify the loading resistors are good on the output side, but even then, whatever mystery amp you're connecting it to would provide some sort of load.
I used to know a great tube amp guy in Jo'burg but he got murdered a few years back, sadly. See if you can track down Glen Turrell in Westville (Durban area) and see if he can suggest anyone to fix your amp.