Nad M3 power board and R, amp

The real techie can diagnose which parts to be replaced and do that. The one full of snit will tell ya to change the whole thing.
Find real one.

Where you’re local to?
Technically, I don't have to even open a unit to blast this kind of diagnosis! They did not provide details. It's not sufficient explaination of the cause.
A power supply as well as output stage consists of circuit elements that need replacement.