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

Not having any credentials such as a manufacturer or scientist, I always refer to the basics.For me it adds even more confusion, because we're talking about a music signal heard ultimately, by ears.

http://www.electronics-tutorials.ws/amplifier/amplifier-classes.html

I took a couple of analog/digital classes in the early 80s. An instructor that was an audiophile would blow the students minds  with endless physics/science  theory behind audio. It was way over my head!

I did become aware that most of us really are listening to just a couple watts of "linear "power. 

Timlub, sorry to do the math for you but 100 watts will gain 3db every time the wattage doubles.  200 watts +3db, 400 watts +6db.

You will be hard pressed to tell the difference in amp power when comparing most a really good 100 watt class A to an average 300 watt A/B.  Check the specs also for dynamic headroom.  That might be your answer.

@elevick 
You are correct,  I just threw out 6db because of going from 100 to 300, but yes, you would need 400 watts for the EXTRA  6db of headroom.
I do disagree to some degree on not telling the difference.  Not necessarily but could be.... I have heard a 40 watt amp that had a huge power supply that was so dynamic that if you had fairly sensitive speakers that you would easily believe that the amp was a lot of power... On the other hand,  regardless of power supply.  If you had a very low sensitivity speaker,  say 82 to 84 db and you liked listening at a decent level, ALLTHINGS BEING EQUAL  you wouldn't be hard pressed at all telling the difference between 100 watts and 300 watts. Of course, you didn't say all things being equal and I do agree that overall, I would prefer a 100 watt Pure Class A amp to a 300 watt mediocre Class A/B

We also need to take into account the output impedance at the rated power. depending on the speaker load, a 100 W amp might play louder if it has a lower output impedance than the 300 W amp, especially if powering a nominally low impedance speaker. 

Take this into account, at normal moderate listen levels, a pair of Mark Levinson ML2 (BJT) monoblocks are only 25w @ 8ohms, but they will out perform things like a Perreaux 3150 300w @ 8ohm (Mosfet) into hard to drive speakers, because the ML can deliver more current into that harder load, where the Perraux will start to compress.

Cheers George