Fender Bassman 50 AB165 Volume and Tone Issues

[pdf64]

The issue may have been that a ‘tin whisker’ had developed. They can be very hard to see.

[Jazz P Bass]

"It was the leg of the pot shorting to the pot shell"
I had a repair like that (A Furman) where the wiper was bent inside the control causing it to touch the shell.

[misterc57]

I forgot to ask about all the orange drop caps in parallel with the filter caps in the dog house. Do not think I have seen those in other Bassman. Anyone know what they are for? Thank you

[Helmholtz]

I forgot to ask about all the orange drop caps in the dog house. Do not think I have seen those in other Bassman. Anyone know what they are for?

Those were not used in original amps. That's a method borrowed from the hifi or rather the HF world. The idea is to reduce the effective ESR at high frequencies. As good ecaps have an ESR around 1R or lower up to several 100kHz and the currents are relatively small, the parallel caps are not required here.
But they don't have any detrimental effect. Actually in rare cases with poor quality high ESR ecaps they might help to avoid HF oscillation.