Always suspect the tubes first. Since your problem is on the dirt channel, not the clean channel, mostly likely it'd be the one labeled "V102" on the schematichttp://www.drtube.com/schematics/marsha ... -61-02.pdfhttp://www.drtube.com/schematics/marsha ... -60-02.pdf
(I have no idea which one that is in the physical amplifier... maybe it's labeled??)
Since you have 3 ECC83's in there, you can just swap 'em around and see if it helps your situation at all. If things change dramatically, time for new tubes. If there's absolutely no discernible difference, then yes you need to take it to a tech.
You might give this cat a jingle: http://buffalo.craigslist.org/muc/4103799210.html
I think he's pretty awesome. Of course, I'm biased 'cuz it's me
Tube amps are childs play compared to most of the stuff I work on. I'm finishing up a couple hot rod Fenders ATM - my bench will be open in about 2 weeks if you're interested.