If you weren't already an old hand at this, I would have suggested going back to first principles and cleaning/checking connectors and volume pots, but I'm sure you are way beyond that. Good luck with the gremlins!
The volume of my speakers going down by itself and going back again in a minute or two
I just upgraded a used Audio Research tube Preamp and a power amp. I found that the The volume of my speakers going down by itself and going back again in a minute or two. What could be wrong? Both amp and preamp are used. I had my speakers for 20 years. Never had the issue before. Help!