74 Silverface Super Reverb problem

[kanderson70]

So I'm working on a AA270 silverface SR and my buddy says the bass will drop out after playing it for a while. I tried it and sure enough after playing it for 5-10 minutes the bass dropped out. I've done the usual check capacitor values, looked for bad resistors, and I'm coming up with nothing. Has anybody experienced this that can offer some insight or point me in the right direction to find the culprit? Sounds like a capacitor issue but everything checks out.
Thanks in advance for any help.
BTW the amp is pretty stock although someone has gone through it and swapped out the electrolytic caps and all appear to be done with correct values and all fall within spec

[pdf64]

Same in both channels?
With the exception of the overall bass level, do all controls still work as normal?
Any other changes go along with the reduction in bass?
A voltage survey in normal and fault modes may be beneficial.

[kanderson70]

I haven't checked both channels. Only the reverb channel. All pots function normally.
I'll give the first channel a listen tonight.