Adding Composite Input (Capacitor Question)

[Electronic M]

If tube subs don't do much and voltage checks don't give an obvious issue (if you have a SS socket saver with test points that usually is the best Best way to verify the socket) then my next step would be to look for carbon comp resistors that have drifted high enough to be out of tolerance (low out of tolerance is usually something else in the circuit in parallel with it confusing the meter).