I'm being spun the story that this was put into storage and on powering up
again , just the mixer section was working.
So the following is hypothetical but is it a possible scenario?
Does not say anywhere that this amp can be bridged, but also does not say it
can't be bridged so perhaps a green light to the owner to run both channels
together.
Anyway blown fuses and some shorted TO3 in both channels. One 16 amp rated
relay had an obvious brown patch inside its casing, opposite the contacts,
but still worked at test levels when the active was replaced. The good
looking one did not click over but its coil ok and audio output up to the
one contact . Both replaced as due to overheating, the plastic linkage from
contact carrier back to flip lever are melted/deformed. The brown stained
one has a serious pitting , well reverse of a pit, sticking out of a
contact.
Now if due to dirt or initial pitting , then poor contact and then making
and breaking current under serious load could that in combination with
speaker insductance create spikes of a few hundred volts to knock out active
in both channels ?
If the bad relay was passing less amps than it should, then excess load on
the other relay and so both overheat.

If DC passed to the speakers , from blown TO3, due to a short in speaker
lead say, then I doubt the situation would last long enough to cause the
relays to overheat from assumed good state previously