Historical look at amps

SS uses feedback to stabilise the circuit. In the more exotic designs this f/back is local. OTOH tube circuits can be stabilised w/out any type of feedback (even there it's not plain sailing -- it requires effort).

Objective measurements: let me insist that measurements are useful and can be reliable. Depends what you're measuring FOR, i.e. it's not always for sound reproduction.

If for marketing, then you go one way. If for guessing the precision of amplified signal, you go another way (i.e. checking into amp-speaker interface matters, et alia).
A Norwegian called R. Lian had looked extensively into this matter. To a certain extend so have we all: if I'm to check for amp performance, WHAT (and how) should I measure?? Cheers

Back to a subject touched by Bombay & Bigtee:

I always thought the "High end" was about accurate portrayal of a source. If it's not and it's about "Good sound" then I need to get out because it all becomes a moot point.

I agree -- yet, much of the time we see "pleasing" prime over "accurate". I.e. many audiophiles are searching for the most pleasing to the ears.
Case in point: lately we sat down with friends to audition equipment. As usual, the commentary edged upon "correct", "better", "more realistic" -- all relating to "accurate" rather than pleasing.
The music was electronic w/ some a'phile "girl with double-bass" thrown in. Quality of recording notwithstanding, how are we to gauge "accurate" or "precise" with electronic music??? A violin, say, (whether one listens to classical or not) offers some easy real-life reference -- as do many other sounds -- but electronic, or even electrical instruments???

Horn speakers, ESLs and magnetic planar speakers all operate according to the power paradigm

... another manufacturer (Mr Pass) also expressed an interest in this and actually played with a number of amp-spkr applications. The spkr units used were wide-rangers, with high magnetic fields (and correspondingly low q -- Lowthers, etc) and the results quite convincing.

A simple point of departure in considering feedback could be the very low output impedance achievable for the amp using (loop) f/back... Now, if we look at an amp alone and we concur that low output impedance is very impressive, then why not be impressed? But then we can likewise be imporessed by its horsepower, looks, engineering, etc.

If OTOH we consider the amp as ONE component of the amp-spkr interface -- i.e. we see it as an operational tool doing a specific job rather than a finality -- then the importance (or not) of its output impedance approaching zero may not be as impressive per se... we would be more impressed with 1st how the combo SOUNDS, and THEN (2nd), what the electrical and design attributes of the amp that sounds "good" are.
For example, we may conclude it's preferable to have an amp with absolute steady impedance fm 5Hz-1MHz, even if we only achieve say 1,5 ohms invariable o-impedance... (actually that's not bad). We might even decide that the keeping phase and minimising signal propagation delays due to loops may be useful: THEN, these (measurable) characteristics, rather than the previous ones, will be "impressive".

In my longwinded way, I'm suggesting that:
*quoted design, standard performance and measurement characteristics currently offered are mostly true and real -- BUT do not necessarily influence the resulting SOUND in a positive manner;
*other design..etc..characteristics may have a more immediate relation to the sound resulting fm amp-spkr interface, and those too are available -- but less standard;
*In older times, when amp+spkrs were part of a single package, there was very little problematic regarding this amp or that, etc...
*Ultimately, as atmasphere has already indicated, it's VERY difficult to design the amp that will do the best with ALL full-range (or otherwise) passive speakers. Indeed, IMO, this item will either be a statistical aberration or ridiculously expensive and experimental, or all of the above.
Cheers