There is nothing particularly outlandish about this circuitry that I can see, fixing it shouldn't be rocket surgery. You'll have to measure supply voltages (I'd expect +/-80-90 V unreg plus +/-20-24 V unreg / +/-15 V reg) and identify whether protection has any particular reason to trigger (notably, measure output DC offset at L201/301 or their respective parallel resistors und confirm it's well within +/-100 mV) or whether the problem is within the protection circuitry itself (if so, suspect C608 first - a 470µ/10V wouldn't have to go very leaky before a 150k has trouble charging it up; the replacement should have as high a voltage rating as you can reasonably make fit, and I would probably throw it on the lab power supply to form for several minutes at least before installing).
The most likely scenarios I would expect would be spurious protection or a grenaded power amp (though sometimes a failed smaller transistor or feedback resistor can also give excessive DC offset without major carnage). See any charring / burnt components anywhere?
