The problem is most likely a manufacturing lemon. Somewhere in that amp either a tiny section of insulation has been damaged, a small piece of wire or lead clipping lodged in a very inconvenient spot, or the wrong value passive component has been installed.
Repairing amps is like fixing cars -- throwing parts at the problem without identifying it is almost always a waste of money. The easiest route to take is to replace parts because if the parts are okay they just put them on the shelf and it costs next to nothing to essentially make a trade. Cary has to take that amp apart and find the bug. But that is a very expensive labor cost, which is why they are trying to fix it on the cheap. You have to tell them to find the problem instead of proposing which parts to change.
Repairing amps is like fixing cars -- throwing parts at the problem without identifying it is almost always a waste of money. The easiest route to take is to replace parts because if the parts are okay they just put them on the shelf and it costs next to nothing to essentially make a trade. Cary has to take that amp apart and find the bug. But that is a very expensive labor cost, which is why they are trying to fix it on the cheap. You have to tell them to find the problem instead of proposing which parts to change.