Fletcher-Munson curve was created many, many years ago and was mostly accepted, and disputed by some. I once built a preamp that operated the changing characteristics of this curve as the volume was turned up/down. And this to my ears seemed to more or less work as predicted.
Very power amps are a benefit with uncompressed recordings, especially if used with speakers designed to cleanly handle dynamic musical peaks. A solo grand piano's need for such power with usual speaker efficiency was once determined to be ~250 watts peak electrical energy from the amplifier if full fidelity of this instrument was to be delivered to the listener.
Even though the System Diagnostic Tool has somewhat limited range, from my Version 5 powered monitor system experience with using a 93 dB sensitivity tweeter directly driven by a 250 watt amp and total SPL output measurement with of all speakers operating seems to more or less correlate with SPL output predicted by the diagnostic tool.
It may be that total power output ability of an amplifier is most important spec to know in real terms. Amp distortion of these amps measured into pure resistance, and not into complex speaker crossover networks is suspect to not tell much useful information other than ideal circumstance distortion characteristics.
Different amps do have different complex load characteristic distortions, and the passive crossovers normally found in 2-3-4 way speakers present very complex drive conditions. So many amps with high power and low distortion specs do not sound as good as others that may handle these varying speaker loads much better. Maybe amplifier peak current output, phase response over frequency, and damping factor is more important to note under these real-world crossover network load conditions.
For this reason, directly connecting the power amplifier to the speakers without any type of interfering crossover network, and using many amps in a system driven with electronic crossovers before the amps has great advantage for getting consistently clean very low distortion amp-speaker performance almost regardless of make of amp if having sufficient power/distortion ratings for the loudness desired.
As with all systems, the quality of the speaker drivers, how they are enclosed in a cabinet, and how those speakers are positioned in a room is the major contributor, and is mostly what will be heard in terms of audio quality.