Okay, here are step-by-step instructions to pull out the chassis and check for and/or fix any loose pots. First, the usual disclaimer about opening up tube amps: Tube amp circuits can hold LETHAL charges even if turned off and unplugged. It's best to let the amp sit turned off and unplugged for a while before opening it up to make sure all the capacitors have drained so you don't get shocked (I often do this kind of thing in the evening when I get home from work). It's unlikely to happen, but when fully charged, those capacitors can deliver lethal shocks. Just be careful and don't touch anything you don't need to...the pots themselves should be safe.
To Remove the Chassis: Remove the aluminum bar from the back of the amp, remove the power tubes (you can leave the preamp tubes in), then unplug all the cords and cables (power, footswitch, reverb, speaker, etc making note of what goes where, especially in the case of the two RCA plugs for the reverb). Then unscrew the screws on the top of the amp and pull them out. The chassis will lower a bit as you loosen these screws, so you may want to hold onto it, but it won’t lower too far, as there are wooden rails there that will keep it from dropping more than an inch or so. Then carefully pull the chassis out the back of the amp…you’ll have to lift it a bit to get it out and it’s heavier than you might expect, but if you lift it a bit it should come out pretty easily…I usually hold onto one of the transformers to lift it.
Checking the Pot Casings: Once you have the chassis out, set in on a flat surface (it will sit nicely on the transformers on a flat surface), then go one by one with each knob, pushing/pulling gently on each one while watching the pot. There will be a bit of play in all of the knobs, but you should not see the pot or its casing moving. If there is movement this pot it could be the culprit.
For any that are loose, remove the knob (there is a little set screw on each knob that you’ll need to loosen up so you can pull the knob off), then remove the nut holding the pot to the chassis. Push the pot back into the chassis and lift it up and out. Then you can tighten the tabs back down with a pair of pliers (you do not need to disconnect the wires, they are long enough to let you work on it with them connected). Put the pot back in place, and put the nut back on. Once you've made sure all the casings on the pots are tight put it all back together in the reverse order. The only thing that might be a bit tricky is getting the chassis lined up so that the screws that go in the top go into their holes in the chassis, but if you line the front of the chassis up with the front of the grille you shouldn’t have too much trouble. And make sure you put the power tubes back in before you turn the amp back on!!.
Hope this will do it for you. If you have any questions, let me know.
