You have asked from a place of affirmed confusion a series of questions
about amps. That one has questions is good, that questions can be posed
about a thesis doesn't harm the thesis, and that one might not accept the
answers does nothing to the correctness of the thesis. The amp that
doesn't exceed it's specs for a given speaker, with regard to current
reserves, headroom, distortion, etc. will most likely not be different
from another similar amp in a listening alone test as to being able to
discriminate which is which. The concept of threshold comes in here also,
various amp artifacts which rise above a certain amplitude,ie. threshold,
can be percieved and those which do not can not; as established in tests
to determine such things. Various types of signal distortion are
examples. The concept often bantered about of "mid high" are meaningless
when the above conditions are met. A "mid" amp that doesn't exceed design
parameters nor produce artifacts above known thresholds can not by
listening tests alone be distingushed from those "high" amps often found
on "reccomended" lists. Amp performance is today a commodity available
from any number of brands which satisify the above. You mention nad,
others have mentioned adcom etc. and all serve as well as those amps with
the famious "high end" marketing reputations and consumers of editoral
inches in the hi fi mags. Design folk have learned how to produce amps
which meet specs and threshold levels so as to make them to be percieved
the same using listening tests alone.