How did I break my amp?

I connected my amp to an old speaker with a somehow faulty crossover, it did the following:
Played back sound “normally” for like 1 second, skip, maybe 0.5 secs, skip, maybe 0.25 secs, went blank. Doesn’t turn on. Can’t measure it since it’s can’t be turned on.

An audio repair boutique thinks the hard skipping behaviour before breakdown indicates it’s most likely the output transistors that blew. Is this quite clearly the case?
The fuse would’ve blown instantly… It does make sense the output stage would skip like that before deciding “alright that’s it”. Since the relay doesn’t click on when pushing the power button, is this the power supply being in safe-mode now so to speak?

Might this have been an issue with the current somehow being oscillated or the crossover presenting an overly reactive situation?
Why might a sturdy Electrocompaniet that handles under half an ohm break down like this?
(Well, it’s actually GREAT it broke, since now the power supply electrolytics will get replaced in the repair, the toroids shielded and KLEI RCAs and terminals added as a nice upgrade! Should I bother with having the gauge of the wiring to the toroids increased for less impedance? Probably superfluous?)

Anyone know where to order custom size mu-metal shield cans for toroids? I know they’re somewhere.

Since a fuse is maybe 50 cents, I’d first replace the fuse.

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Oh I’m gonna have it changed to an SR fuse alongside other repairs/upgrades. I want this 20-year-old amp to its max potential, good thing something broke since I would’ve waited longer for the upgrades otherwise. Won’t be doing them myself, the repair boutique takes about 300€ for the repairs + additions. Wouldn’t trust my novice solderhand with a high-end device like this.

Then again if I can get it working asap with a fuse change, I wouldn’t mind…
If the output stage is about to go, will the fuse likely fail first in a well-designed amp?

20 years old? Replace all caps with the best you can afford.

It’s obvious to me that each and every electrolytic should without question be replaced, but really, would fairly high quality / “good enough” polycarbonate or polypropylene caps degrade in 20 years of normal usage?
Surely they can’t be invincible, not what I’m implying, but really, is there actually going to be major breakdown of their structure in such a time?