Crackling could come from a number of things. Dirty jacks, bad cables, dirty pots, bad tubes ect. Before yanking any tubes I would check all your cables and clean your jacks. Check any patch cables/jacks on any pedals if you are using any as well. You can plug the guitar straight into the amp to rule out any pedals or patch cables and try several guitar cables just to rule that out to. If all that is good and you know your guitar's electronics are good an clean, then i would move on to pre amp tubes.
