I wouldn't write it off completely just yet. Did you take the bottom cover off and check those fuses too? I could tell you to check certain components if I had one of these in front of me but I don't so I am just going by the pictures I can find online. It looks like a relatively simple to repair unit if we are talking just the output stage.
I think this is the internal view of that unit.
If the problem stops with the power transistors, that is one thing. Sometimes it does, sometimes it does not.
Even so those power amp boards are pretty crowded, so you would need know how to find your way around.
Personally, I don't usually get very far servicing gear like that without a circuit diagram and better still a service manual.
I can't find either on line.
I doubt this is a fuse problem as the protection relay is cycling, indicating the power transistors are getting power, but shorted, with an excessive collector to emitter current draw. The relay is protecting the power supply.
The OP has had an invaluable learning experience. This unit was given to him, as even two years ago there was not much value remaining in the unit. For around likely double the cost of repair, he can get a better unit, that is up to date and offers more facility, like streaming.
If he has enough skill and experience to fix it himself fine, but absent that my advice to to recycle it and move on.
