Sorry to bump an old thread, but I had an issue with my old HH amp that suffered the same problem.
HH used weird inverse-log pots with odd valued (27k iirc) that would be tricky to fix. The common consensus is to use contact cleaner. The problem is that contact cleaner makes a 'dirt soup' for a while, then dries leaving the same crud in new locations.
To solve the issue I carefully took the pots apart starting with the little tags that hold the metal back over the front part of the pot with the lugs on it. Once in bits, I cleaned up the carbon tracks with a pencil eraser. They changed colour! It was amazing how much rubbish came off. Contact cleaner would be no use in that scenario. I reassembled the pots and the amp works perfectly!
Electra/Westone Club #19, Guild Club #27 (snuck in with a Dearmond).