"That was only ever a problem with tube (valve) designs. "

That is only partly true. Solid state amps that have output transformers or chokes are also vulnerable. In fact Delco car radios had a warning sticker on them that read "DO NOT OPERATE WITHOUT LOAD". Those were a class A open ended stage simply running into an inductor, but it did produce more power than the SESAPP at a given voltage. It was actually capable of almost nine wattts nto eight ohms but they didn''t bias them hard enough to hit that level without distortion.

Any amp that uses an outpur transformer or choke can be damaged if operated without a load. The old tube stuff, what was actually the problem there was the tube sockets themselves because of the unbridled inductive kick when they got clipped with no load.

I've been in the argument about tubes and don't want to dwell on it, but one thing to remember is that a triode is still a diod, a tetrode is still a diode. When you get to a pentode maybe not so much because of the suppressor grid, but most of the other ones simply do not conduct in reverse. Generally the output devices and transformers could handle the voltage, what happened alot was that the tube sockets arced over.

The failure mode is different in a solid state unit. The inductive kick causes forward current to flow in the collector to base junction and the transistor really does not like that.

From what I've read a bit more recently, because there are still tube amps around, especially for guitars and these high MFs might forget to plug in the speaker, (oh, I got tons of stories about musicians, I mean, nice guys usually but electroncs is like speaking Martian to some of them) now some designs include a diode to ground from the plates of the output tubes. This limits, actually damps, the inductive kick in case a speaker is not connected. However one tech around here reported that he had some problems adding such diodes to an existing design, and I never got the full details on that. It's just not something I have explored and queried about.

Anyway, getting to the OP here, the main thing is simply to look at the unit and see if it uses an output transformer or choke. If not, most solid state equipment can run full blast for decades into no load. However, not knowing the impedance of the guy's siren or whatever, there might be a step up transformer to drive it. Actually that would tend to be less susceptable to such damage than an output circuit that steps down.

When they got to OTL (output transformerless) SESAPP (series arranged, single ended push pull) circuitry the danger was pretty much gone unless the design itself was defective. Or people. I remember working on a Harmon Kardon Citation 12 and people said you could use 2N3055 for outputs. NOPE. The voltage Vcb is not high enough. Now in THAT case because the load is not pulling down the power supploy rails there might be a prblem, especially iif you have transistors that are margnal. Those ratings on the spec sheets are MINIMUM, or WORST CASE. Personally I just use 2N3773s in something like that, which have a nice smooth gain curve as well.

Bottom line, most stuff doesn't care if there is a load. Exceptions are far and few between unless you deal with old stuff or tube stuff. This is supposedly an alarm system, and those barely existed in 1963 when this could be an issue.