Another source of crackling to look at, is the aluminum foil on the amps. This foil is supposed to shield RF and other extraneous noise. But most times all it does is cause unwanted noises. The foil does not make good contact with the chassis and will cause crackling that can come and go. Loose components can also cause some crackling or even the signal dropping out. Some components to look at are pots, jacks, transformers and mechanically connected grounds.