I need to get a louder amp and have an opportunity to buy a used GK 700RB at a good price. I haven't played on it yet, but the present owner says that it works fine, but the pots may be dirty because it crackles a little when you turn some knobs. Does this sound like a dirty pots problem or something bigger/worse? How much do you think it would be to fix or should I just stay away. It doesn't crackle when playing, just when adjusting knobs. I am trying to decide if I offer a lower price subtracting out the fix cost or just walk away from a bad deal. Thanks for your help.