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

@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

I've got sitting on my table a clone of a First Watt F5 running hot rails of 32.5V and biased up to 1.26 amp. In push-pull, that's 2.52 amp by 32.5V for a class A envelope of about 82 watts. 41 watts RMS. Keep in mind that when you halve the load impedance the amp will double it's output, but it will also halve the class A envelope and push it into class AB operation sooner. 
The power supply really is the crux of the matter. A muscular class AB amp might only call for 50,000uF of filtering, and may only require a 600VA of transformer. I think that's what the Schiit Vidar is packing. I've seen guys building F5T amps with dual 800VA transformers and and 480,000uF of filter capacitance. My F5 has a 600VA toroidal and 120,000uF of filtering for 41 watts output stereo, and there are guys that build that amp with one of those supplies for each channel. Class A amps require absolutely massive power supplies because they're always pushing the full class A output power through the circuit at all times. You'll never find class AB amps with power supplies like class A amps. The class A envelope on mine is only 82 watts, but it will transition to class AB and handily push over 300 watts RMS until something melts. As it is, it barely ever leaves class A driving my Focals. 100 watts in a well built push-pull class A amp is an absolutely massive amount of power with huge headroom, likely far more than any class AB amp of triple the rating. 

Mastersound PF 100 Monos. 110 class A SET. 
Quite an experience. Take a control over the speakers unseen for me before.  The only amplification I can confirm that kef reference are musical. 
Simply amazing