I think you have a bad capacitor in the driver stage of your amp. That cap looks like this,

, and a good one looks like this

.
Maybe something fell in there, clean it out with a vacuum. Bad caps don't start their cracklin until it gets hot and warmed up.
Look for expansion on the top of electrolitics (caps), looks like a bulge, or look for fluid leaking from one.
Trouble is, a bench tech has to check all them caps to find it, unless it looks like this,

, then it's in and out.
Hope this helps,
--LTL