For investigating the PA section it is possible to detect faulty parts by measuring voltages in circuit so leave them 'in situ' for now.
I find it convenient to use Parrot clip test leads and to fit these with the power off - in that way the hands can be free to write down the voltages on the test part for each circuit designation on a paper copy of the circuit diagram. In particular I would be looking to see the voltage difference between base and emitter is about 0.6 - positive for NPN and negative PNP.
If you take this approach the light bulb can be dispensed with for fault diagnosis - the damage has already been done by a previous owner.
Since the parts at the greatest risk of failure are the output and driver transistors, the shortest route is test these first working from the output backwards to the input. There is some protection for the small signal transistors that shuts off their current supply if the output stage is over-driven. I would be surprised if the damage extends beyond the output and driver transistors.
If there is a need to replace any of these you will need some safeguard in place when you power up again - the light bulb. I have a pair of 100R 5watt resistors with alligator clips attached to either lead.
I fit these across rail fuse holder in lieu of the fuses with my meter connected to measure the voltage drop - and from Ohms law the current drawn. You can use this to get Iq in the correct range for final adjustment when the fuses are refitted.
As far as out of circuit tests go an NPN transistor is like a pair of diodes with the positive connection for each tied together forming the base and the cathodes being representative of collector and emitter.
It is sometimes possible to identify a faulty power transistor by comparing the diode junction behaviour between the same designated part on the neighbouring channel. It is easier to remove the leads of a faulty transistor from a board with the plastic body cut off.
Some years ago I built an in-circuit transistor tester from a magazine project - this has been very useful over many years.