Many times the black Nippon Chemicon caps in old Luxman are pretty good. The main amps are prone to failure, or loud hickups because of deteriorating transistors, but if it behaves correctly the "12 o'clock" thing MIGHT be overloading the amp only. Most amps are pretty loud at the 12 oçlock position so whether it is bad or not, first listen for yourself before making the same conclusion as previous owner.
In the beginning of the service manual they write 50 milliamps a s bias but this is rather high. This would be some 25 millivolts over one of the output transistor emitter resistors.
If the voltage is over + and - 50 volts the 50 milliamps bias would already result into 5 Watt dissipation each side. So the amp will be running warm on the back side even idling only.
Myself, I made sure my SQ507x and L309V went not over 40 mA whether hot or cold. One can carefully heat up the amp back side using a heater or hairdryer, but take care not overheat anything. A dummy load or having no neighbours and cranking it up is better for heating....A temperature probe comes in handy, or assume if you can not press your finger against without pain it is 55 degrees celcius or more.
IMO for distortion there is nothing wrong when bias is just 20 milliamps or so, maybe even less.
Personally, I suggest determine whether the amp is at fault, at all. BTW yours looks pretty clean, a lot better than this picture of the detriorated amplifier cards of my L309V before refurbishment.
The R1500 is a real nice receiver, mine is still on the pile of unrestored stuff (not much time for it)