"There are no class D amp chips and the mode of operation of the output stage
is mysterious. "

Not really. That is only a one channel amp. Even operating in BTL, if you sit down and figure out a class D drive circuit using discrete components, the count is not all that high. Class DB might take a little ore but since this is a bass amp the filters can be miles away from the operating frequency so DB is not really all that much advantage. Did you stick a scope on it and find out just what the amp itself chops at ?

"** I have sometimes added a Zobel ( ie 22nF and 56ohms, 0.5W) ) across the
output of an amplifier that did not need it.

Why ? As a tell tale in case of supersonic oscillation. "

Good thinking if you suspect that. I take it you mean mainly in analog amps..

I have done similar in amps with fuses internal. When they do not have current limiting amd people stick like a 2 ohm load on them and ****, it ain't my responsibility if the waste more silicon and they pay. I almost never do that, but have in cases of multiple recalls. One multiple recall was due to a leaky muting transistor at the input of the power amp. I ALMOST got to accusing the customer but it was always the same channel. The boss said they shouldn't be turning it all the way up, but I disagree. They are entitled to use every last watt that amp can put out. Since it was after the volume and tone controls the problem would only show up at very high levels. The ****er was rectifying the audio and imposing DC on the output. But that was where everyythignweas under warranty, a rental outfit. there is no simply siying screw it and give them a refund to divorce a piece of junk. We would have had to actually replace it. Another company found that out about selling service contracts. If you pay your housde insurance and your house burns down, just refunding your premiums does not cut it.

"Incorrect installation and bad cabling practices can cause almost any power
amp to break into oscillation at an inaudible frequency resulting in rapid
overheating and failure of output transistors etc "

This just happened to a buddy of mine recently. It was making a funky kind of distortion which cleared up at higher input levels. I said "Shut that off NOW !". Tunred out a speaker wire was running parallel to a line level input, but that was still downstrean of the gain control. It seems that luckily, the oscillation was high enough not to fry his tweeters. I cleaned that thing up and it is now quite happy, except for the one amp working into four ohms.

"Maybe you have never seen it - it kills output stages in 30 seconds"

Think about what would happen with piezo or electrostatic tweeters. Even ribbon tweeters. When it happened to my buddy, it must have been high because if it was only 50 kHz or so, with the power, those tweeters should heve been toast.