re the OP.. crackling + some fuzziness without volume/power loss - many poss culprits..
P/tubes likely, preamp tubes less likely, a leaky coupling cap poss but not that common, then many other poss causes.
It's chicken or egg whether one chases the actual fault, locates/proves/fixes it.. OR do routine maintenance only to find the fault is fixed in the process..
The pedantic or more thorough method is do the fault first.
Then you KNOW what it was and is fixed..
Rather than fix it with routine maintenance(you think) and have the amp return because symptom returns later and proves U didn't actually fix it in the first place..
For your particular symptoms, I agree cycling pre and power tubes is a good start.
FWIW, your original post took my mind immediately to power tubes.
After that, take line out from fx send - it it fuzzy in the slave amp?
Equally, inject known clean signal into fx return - does it get fuzzy in the boogie ?
Let us know how things progress. regs, Dave