I had noise problems with mine when I first got it, and solved them by tightening the screw between the tubes (removing the tube guard first). Since your problem is channel specific, that may not help, but it’s a quick thing to try.

If turning the knob really increases the noise, dirty pot could well be the culprit. I’ve never cleaned a pot on one of these, although I’ve cleaned them on amps plenty of times, and it’s not particularly dificult, just swab the contact with some rubbing alcohol, and let it dry.

Taking it apart to get to the contact, well, that’s the fun part…