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

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..

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.