G'day mate, I've just discovered this thread and it reminds me of my battles with my now quite deceased NAD 3155 integrated amp. It started tripping the protection LED one afternnon without warning and blowing output transistors.
An experienced repair tech I took it to, said being a DC coupled amp faultfinding was practically impossible and he said it wasn't worth spending time on, and that nearly broke my heart. In the end, I built one of the Elliott Sound Products 'chip' amps which is very nice and was quite easy to build!
Be aware also that the heatsink compound used on the output transistors may be bellylium oxide based, which is quite toxic. Take care with that stuff. I believe it was commonly used in most amps from that period. Regards, catman.