If the amp hasn't been serviced in a while, the jacks (input, output, send/return), tube pins, and socket contacts should be cleaned with Deoxit. The pots can also be lubricated with this product. A tech would normally do this as part of a standard service.
I think that the first thing to check is the preamp tubes. The fading volume is a classic sign of a bad tube. If you can't test the tubes, swap out the pre-amp tubes, one tube at a time, starting with the first (input) stage with a good replacement to determine where the problem lies.
You might find it helpful to read this
and see if any other symptoms are present. All you need is one though.
If that doesn't resolve the problem, it could be a failing component or a bad solder joint. A tech would be able to help with this.