Ok so recently I purchased a used Peavey XXX Head to go with my 5150 cab. About two months after the purchase, a crackling sound became evident when I played, almost always while palm muting. I just bought new tubes thinking that would cure the crackle but it didn't. I've used different combinations of cabs and guitars to isolate where the problem is so I'm 99% sure it's in the head. Any suggestions?

Yeah but that didn't affect the crackle at all. It was there before I replaced them and it's still there now :/
Have you rebiased for these tubes? (Doesn't sound like this will help but beyond that I'm unsure...)