There are a couple of possibilities here. The crackling is likely to be a dirty control. Spraying it out with electronics switch cleaner that contains a lubricant should resolve that. The popping usually indicates a build up of DC current between the amp stages. This can happen because of defective coupling caps. That may take a little more detective work. Try cleaning the controls first. If that fails to reolve the entire problem, update this and we'll start looking at circuit components.

Dan