Swap out amp or preamp to determine which piece is giving you the problem. If you are not a tech, then you're going to have to send that unit in for service. Could be something simple like oldhvy mentioned, worth checking.
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!