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

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 

Gentlemen,

Unless you are using a sound level meter to determine the actual sound pressure level what you are probably experiencing is the following phenomena which is that we think something is playing louder when it starts to distort and don't think it is too loud when it is not distorted.

Under circumstances of no instrumentation to aid your perception the 100 watt amp sounds louder because it is distorting sooner, the 300 watt amp sounds less loud because it can play louder with out distortion.

Your brain and ears are deceiving you..