A bad preamp tube usually will let you know via a microphonic screech, distortion, lower output. While it is possible the op has a couple of bad tubes, the chances are unlikely. Unfortunately, scratchy/dirty pots are a reality and are an easy fix if you have access to them to spray some Caig De-oxit or other types of contact cleaner into them. I prefer De-oxit as it is also a lubricant.
Jim
