I don't think Jim has any semiconductor diodes in his amp, but he definitely has resistors. I've seen lots of old gear with cracked potentiometer substrates, and they'll make noise too. It crackles when you rotate the knob and cleaning doesn't help. The resistance changes through warm-up, it makes and breaks connection and causes a lot of noise. So I agree with you about resistors, both fixed and variable.

I'd look there before I'd look at caps, which normally fail open or get lossy. Neither condition usually makes the symptoms Jim described. Might look at mechanical connections too, like tube sockets, switches, potentiometers and what-not.