How old are the tubes 5 years? 10? If they are 12AX7 or known in the UK as ECC83 just change them out with new. They're cheap. Lots of good brands. Determine which tubes you have by the numbers and or labels on them.
Here's some links that you might find helpful.
http://trdpartsonline.com/index.php?mai ... cPath=5_73http://www.tubedepot.com/I have JJ's in my Marshall in the preamp, Ruby EL34's for the power.
In my Fender type clones I have JAN Philips, Sovtek, Tesla, and Electro Harmonix. All good tubes. I should add that while in the past I have been against the use of Groove Tubes, I am told buy one of the best amp techs in the business they have a pretty solid lineup and overall good quality. He sold me on them and I just might try them.
I won't tell you straight out that the problem is with the tubes completely but usually, and I stress..USUALLY, that is the case. Should you swap them out and that is not the case, take it to a good amp guy or do one better and tech it to him now before buying tubes if you are a little strapped for cash.
Got any pics of that amp?
Cheers, TB.