Customers are strange. Some get the idea you'll fix the amp really quick if they call you all the time, and will put it in the back yard and forget it if you don't. Others don't seem to want them back at all. Pressures in their lives get transferred to the repairman - but actually that shouldn't be so.
Why I'm writing is not to moan about customers but to say that I had a Laney with this problem and it turned out to be a coupling cap that looked as if it was soldered in but actually the wire hadn't gone through the board. I see you've resoldered everything though.
Input jack? Dodgy pot?
If you can feed a big signal in and make it crackle you could probably see it on a scope and trace it down to its origin - or use an audio probe and listen for it if you can bear the buzz. Audio probes are good, I have a nice old Emrad ss jazz guitar amp in at the moment that hisses too much; hard to see hiss on the scope but the audio probe picked it up and I traced it to its three different sources (a dropping resistor, a zener and a transistor) easily by listening for drops/lifts in hiss across components etc.
Generally opamps that go wrong either hiss/crackle all the time, or just die.