If there's a valve rectifier (especially indirectly heated like here 5AR4), I suggest to just ignore or eliminate standby; none of the typical arrangements are downside free or are anywhere near evenly balanced (in regard of technical downsides and benefits).

It's in its 3rd or even 4th decade of use / abuse, probably single sided, non through hole plated pcb?
Lots of failing solder joints are to be expected.
The symptoms match up with that.
Ideally inspect the board joints first, using visual magnification and dyrong lighting, to try to identify the culprit/s. Those for the sockets and pots are typically under the most stress.
Then reflow every solder joint in the thing, remove old solder first, especially if there's an excess.

The circuits around IC5A & B look like gyrators (rather than low pass filters per se) to me.
Resonant filters.
There's probably online calculators to derive their response, and teemuk's book will have the circuit explanation / analysis / equations.
https://www.thatraymond.com/downloads/solidstate_guitar_amplifiers_teemu_kyttala_v1.0.pdf