/diy/ - Do It Yourself

I asked this in /sqt/ but was told I'd have better luck here:
I have an old amp I got used for cheap. I noticed some of the componemts get very hot with use, specifically two transistors. They are the ones marked in red in the schematic. After using the amp for about an hour, their heatsinks are so hot that I can't keep my finger on them for more than a few seconds (I have no other way of measuring the temperature, my thermometer only goes up to 43°C).
Some more pictures:
Whole Amp: https://0x0.st/i-Hf.jpg
Q801: https://0x0.st/i-HV.jpg
Q802: https://0x0.st/i-HW.jpg
Top case: https://0x0.st/i-H4.jpg
The bigger of the two discolorations is above Q801 and the smaller one is above Q802. Both were already present when I got it.

I did replace the thermal compound on the main amplifier chip (IC701, a STK4191) and on Q801 when I was cleaning the amp (it was full of dust) so that can't be the problem (also, it's the heatsinks that are hot, not only the transistors themselves)

Is it normal for them to be so hot, or should I be looking to buy some replacement parts? Do I maybe just need bigger heatsinks?

Not sure whether I should as here or in /ohm/, but can anyone tell me the two transistors marked in this schematic do? I'm asking because they tend to get pretty hot when in use and I wonder if they have some kind of defect and need to be replaced.