"JamesG" top-posted:

That's what I do for my guitar amps. I place a 100 ohm resister across the
output. Cheap insurance.

What about a much-higher-than-speaker-impedance resistor? 10X? 20X?

That's a wise thing to do with any tube amp.
I use 68 ohms/2 watt carbon composite.

Someone who shall remain unnamed for the sake of common decency,
posted another good advice: in each tube anode leg 3 or 4 1N4007
diodes in series with the anode to ground.

--
Sander deWaal
"SOA of a KT88? Sufficient."