How does a tube amplifier react to a short circuit or interruption at the speaker connection?

With this question, the main distinction is how high the output signal is:

With no signal or only small signals, usually nothing happens in either case
With medium signals, a short circuit usually does not cause any problems, but an interruption can be problematic
With large signals, the short circuit should trigger a fuse; the interruption becomes particularly dangerous when distorted signals, such as with a guitar amp, are involved. Unfortunately, this often leads to arcing in the tube, at the socket or at the output transformer, which can be expensive
In many tube amplifiers, it is therefore not a problem to briefly reconnect the loudspeaker without a signal
Very rarely, there are also devices that can start to oscillate without a load, so caution is advised
An output transformer almost never burns out just because a speaker becomes very low-impedance at a certain frequency; semiconductor amplifiers cope with this significantly worse