You would need to trace the AC signal voltage through the circuit. If all the panel controls are working correctly the problem is likely after the preamp, so that would narrow the search. The crackling could be indicative of a bad solder connection, a bad pot, a bad tube, a dirty tube pin or socket, etc. Unless you have a signal generator and a scope (or at least a DMM) you would probably be money ahead to take it to a pro unless you WANT to learn this stuff. Which can be fun and rewarding on it's own. But if you just want a working amp a shop may be your best bet unless you have electronic bench gear and some chops because the symptoms at this time are vague without bench tests.