I don’t believe it’s clear what the problem is and therefore the cause. It’s easy to interpret a fault incorrectly.

Pots are normal either on or off if the tracks are worn.

Capacitor however if leakage is present can play up. If the value went down it would account for less bass output. Two amp inputs halves the input impedance and that raises the -3 db time constant of the capacitor. If that cap it bad a parallel connection will only make it worse. If it’s the power supply cap then the active parts of the circuit may not function correctly.

It’s a simple service issue for s tech with an oscilloscope.