I know its not my amp because I've tried different basses on the amp and it doesnt crackle. Its a small Rumble 15 amp and the bass its crackling on is a MIM Fender Precision bass. Whenever I play a note it crackles ALOT and very loud. I've messed with all the knobs on the amp and bass. I think it might be these two springs that were under the pickups that came out when I was replacing some screws but i doubt its that.
1) Bent output jack 2) Loose wire on the jack 3) Loose wire on the electronics 4) Dirty volume or tone pot (exercise the pots) Put the springs back. They're in there to adjust pickup height.