Look at the basic power formula.
P (power)= E (voltage)/ I (current)X efficiency (50 to 66%)
E is 2400 (guessing)
I is 500ma Those old Black cats are weak on their HV PS.
(2400/.5) X 66%
1200X 66%= 792 P
So roughly around 800 watts if driven within specs.
Some of those amps have a driver tube, others do now have.
Keep in mind if you drive it easy and get a cool clean 500 watts out that old amp may last you many years.
Components have about reached their life span as far as usage, 20 to 25 years old, caps etc. etc.
You may want to take it to a good tech that actually has test equipment capable of checking electrolytic caps ( not just a multimeter) and update the HV ps components.
Those Old 3-5 are getting expensive to replace so drive them easy and they will last you for years. Over drive them and they can get expensive.