Yes, physics dictates how well the electronics chain works, especially with regard to the power amplifier.1. Physics has to do with everything
2. Amps don't have a volume absent a speaker interface. Perhaps you mean output.
3. How do you measure "incompetence"?
I obviously meant that in the combination of speaker and amplifier functioning at high volumes, that typically the amplifier will be the weaker link, in the areas I consider to be important.
To "measure" incompetence would require monitoring the output of the amplifier while driving the speaker at higher power levels; yes, this is already done to derive numbers, but the numbers are not related to what one hears. Subjectively, the term usually applied is "effortless" - a competent amplifier is asked to deliver more, and the sound gets louder - and nothing else happens; for incompetent units the tonal colouring starts to change, transients lose their bite, the treble begins to degenerate, "compression" is heard kicking in; the impression is that the system is under stress, and you don't want to keep listening at this level.