What I meant by "a cascading failure of a large capacitor" is that it will always continue to get worse: The leakage current will continue to increase, the heating will always get hotter, and the failure will continue to the point of a total short circuit.
Thank you very much for your answer, and apologies for the delay.
I've checked and found no solder bridge. Nevertheless, one of the female sockets on the board for a power cable (next to the fuses if I recall correctly) was very desoldered (the pins were floating so probably some of them were not in contact with the pads).
Some weeks ago I decided to get another 2nd hand amplifier (this time a Sony str-k780). This way I'm not under pressure and can properly try to figure out what's going on. About shorting the speaker output, I'm not sure how I could probably have done that: I marked all the cables, so I suppose I didn't mix them, but as I was quite stressed I cannot know).
I will check the rectifier as you mentioned, as this may be a big candidate.
The main power rectifier in an audio amplifier is one of the more highly stressed components. Thus more likely than a lot of other parts to fail. Filter capacitors likewise with ripple current constantly flowing in and out. So physically larger capacitors may have less internal heating. So if the voltages are close to correct but there is to much hum, capacitors that have excess internal resistnce could be the cause..
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