I have no experience with NAD equipment so the following applies to solid state amplifiers in general.
I think perhaps you did not fix it. Most solid state amplifiers are self biasing, they require no adjustments. If it is overheating after a few minutes then there is still something wrong with it.
Without a service manual or at least schematics it will be difficult to repair. The usual parts to fail are the output transistors and the emitter resistors. Beyond that you really need schematics at a minimum.
