At least try another ECC83 V3a/b. Lets eliminate it.
Reverse log or anti log pots are scarce. Usual dodge is to make a 'fake anti log pot' from a linear pot.
Find a 500kΩ dual linear pot that fits the S-99 hole and knob.
Use a 47kΩ (or 68kΩ) resistor to connect the centre tag (out to vol control) to the input tag (from the Normal / Reverse switch). One for each half of the pot.
It will work better than what you have (had). Have a look at
https://sound-au.com/project01.htm#s5 Better Balance Control fig 8.