Resistors and capacitors tend not to fail easily unless they're dissipating enough power to get hot or have lots of voltage across them so they wouldn't be the first thing I'd check. On the other hand, switches and pots do get dirty and can be problematic so I would give them a squirt of lubricating cleaner if you haven't done so already. I've known horrible sounding amps to come back to sounding like new after a simple switch/pot clean-up.
Worth a try before tackling anything else.
That was the first thing I had done before starting the whole thread. I'll put some more in through that tone defeat swicth again though Guess I'm looking for advice for some spots to touch down on the audio tester to see where the signal might be getting lost (though it never really gets "lost" it still gets through, just isn't being "processed" correctly...