I suspect its use of SMPS type powerfets in the audio side of this ,
IRFP240/9240 pairs rather than audio grade with whatever is inside of
those to reduce chance of oscillation in // use.
Anyway replaced those 2 and running with IRF740 in the smps , 60% mains,
then put in 2x STW33N60 and amp running ok at 100% mains, fingers crossed.
Mods were 20 amp stubs of wire from heatsink to ground pins of the 2
33uF,100V 105deg F caps, I uprated voltage on the replacements as +/-82V
unloaded.
Added 180pF over the 180pF of C2 to reduce f spec of the amp, bass amp
anyway.
Aded back to back 4.7V zeners over the back to back 6.2V zener gate
protectors of the audio amp, as owner only goes to 60% volume.
Added blocks of cured silicone sealant in the easy air paths forcing the
air through the heatsink vanes.
For the replacement 240 and 9240 ground back the flange bits of the gate
pins to take 10mm long ferrite beads, unknown spec 4K ohm over ends.