What are the weak points of Pass amps
Earlier Nelson Pass Amps, particular Threshold used Bi-Polar output stages that had far higher current ability, which allowed them to drive lower impedance speakers without strain or current limiting like todays Pass Labs Mosfet output stage ones can.
But Mosfets are more "bomb proof" than Bi-Polars, if the owner did something stupid and momentarily shorted the output or overloaded the input.
Cheers George