How could 100 Watt class a has more head room than a 300 Watt amp Class AB

'Headroom' can have a lot of meanings in the audio world...

At system-level you may think extra db from the average listening level the system can play before non-linearity/distortion, for example. This does involve the amplifier, but all other elements also including acoustics.

Headroom for amplifiers can mean extra power capability above rated, typically for shorter peak periods. But that's not all the possibilities for the meaning.

If you are using speakers that reach down to low impedance at some frequencies (as mentioned by georgehifi previously), power supply / output stage current capability and output impedance may be way more important that rated output power. First Watt Class A is a great example of that type of amp that can drive low impedance easily w/o fatigue on difficult loads.

But it all depends if that's a factor in your particular speakers/system/listening style. Really. An apple may not be the right fit for a person who is looking for an orange...

Unfortunately there's no general rule of thumb figure, it's all situation-dependent.

 The sound of music (wasn't that a movie?) when it reaches your head is far more important than the wattage your amp produces, so if it sounds good at 12 or 2000 watts, it's good.  A Class A single ended amp gets rid of whatever crossover distortion might exist in push pull circuits so there's that, and when exposed to this phenomena the listener is rendered a better person almost instantly. 

A Class A single ended amp gets rid of whatever crossover distortion might exist in push pull circuits so there’s that, and when exposed to this phenomena the listener is rendered a better person almost instantly.

+1 for that, and that’s any class-a not just SE.
Ever listen to a pair of old ML-2’s in good working condition through good ESL’s with maybe dynamic bass like Monoliths, MAJESTIC!!!!!! So long as you don’t want party levels.
This amp keeps doubling down to 1ohm, I’ve seen it do it on the bench.
Cheers George

ML is in the past, Gryphon is in the present. Bloody expensive stuff, though.