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

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..

You've got a point, if we're here talking apples to apples, but where not. We're talking totally different classes of amplification. My 41 watts class A amp will transition to class AB and push WAY more power than 41 watts. It can reliably bang out about 150 watt peaks for brief before reaching the limits of what the transistors will tolerate, and that's only really limited by the sinks accumulating too much heat. Yeah, the distortion does get a little high, but not horribly so. A 100 watt class A push-pull should be able to effortlessly drive about 300 watts into a load without burping. 
But think about that kind of power for a minute. 100 watt peaks are MASSIVE on half way efficient speakers. Unless you're driving horribly inefficient speakers in a mammoth space, are you using anything like 300 watts? My 41 watt F5 practically never leaves class A regardless of how much Bassnectar I throw at it. It idles at 54C no matter what I throw at it. 

To compare meaningfully output power of two amplifiers (all external factors being equal) it is useful to know several parameters.
--the manufacturer's definition of the power output metric. It should be
   "continuous power", meaning the amplifer's delivery  is not time limited  and it does not overheat in the ambient temperature 
--what is the distortion 20hz-20khz
--what is the signal/noise ratio
--what peak output power can be reached at 2-4-8 ohm, for how long, at what frequency and with what distortion
The peak parameters are useful to know, as musical peaks may need 5-7 times value of the continuous output power.