I could be way off base, so consider that. But you have both 15v rails at the zeners, but do you get both rails right at the IC, IC8b?
Just thinking, but normally that IC sits between the two 15v rails and centers between them. Well, the center between plus and minus 15 is zero. And whatever appears coming out of that IC is what is going to output the amp. At least at idle.
SO what if one of the power rails doesn't make it to the IC? Then the poor thing tries to center between 15v and zero, which is 7.5v, and that is what is on the output. Obscure, yes, but possible.