Loud crackling on several pots

Sounds like your pots are dirty. You need to spray a cleaner such as Deoxit in the pots then rotate them a few times..This has to be done with the power off and the amp unplugged from the wall...Remove the knobs, get some food grade plastic hose which will be a snug fit when shoved on over the shafts of the pots...make sure to cut a piece off the hose about one inch in length...spray the cleaner into the hose and then with the hose still shoved down over the shafts rotate the shafts back and forth a few times...Then power up the amp and recheck...This will only work if the pots are not gone beyond the point of no return..Outside of that, they will need to be replaced which means you will have to disassemble the unit...