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


Put aside which brand or make.
I put two amps into a test, both highend amp came from the same manufacturer.
Both double down the power with half of the impedance load, and THD is about the same.
Regardless of the size and cost difference, from a pure science perspective.
300 watt in theory should provide more headroom and sound ease when it reaches 100db, but the reverse is the true, the class A 100 watt seems to provide more headroom.
I have tried to use another set of speakers which is much easier to drive and it reaches the same conclusion.
Can someone explain why?
Quality or quantity of watt, how do we determined?
samnetw
I'm with @tablejockey 

Not really convinced that your speakers ever even drew 100 watts.  

You can actually measure how much they're pulling at those volumes using this technique:  https://forum.audiogon.com/discussions/how-much-power-do-i-need-find-out-using-this-method
From what I've seen in the Stereophile reviews the Pass XA series amps are rated for their class A power and not peak power capabilities. It looks like they tend to double when run up to the limits of the power supply.

From the Stereophile XA60 review it'll do 130 watts into 8 ohms and keep going up as the impedance is lowered. It's just running in AB at that point. The A rating is probably limited by heat dissipation and the full power output by the power supply.

From the Stereophile XA60 review it'll do 130 watts into 8 ohms and keep going up as the impedance is lowered.

130W into 8 ohms 
210W into 4 ohms
330W into 2 ohms

It's no wattage doubler as you can see from the Stereophile tests, but it at least it keeps going up, some like many Class-D's just s**t themselves into 2ohms, and can't get much above the 8ohm watts.

Cheers George