Shadorne, the majority of amplifiers lose low level detail of frequencies in the mid-range, i.e. 250Hz to 500Hz, and that is why people crank it. It has very little to do with the frequency extremes.
I agree with some your comments pertaining to phons, but you would need very sensitive ears to hear 20Hz. Very few speakers will produce sound under 30Hz, let alone 20Hz. Personally I feel low frequencies in my stomach before I actually hear them and we probably hear the harmonics of low frequencies without hearing the fundamentals of said frequencies themselves.
Same with frequencies of over 15KHz. I doubt many males will hear anything over 18KHz.
If the midrange is incomplete, the frequency extremes do not matter. Incomplete midrange = inferior amplification.
And forget about the theoretical dynamic range of CD. The majority of CDs are compressed to under 20db range. I listen mostly to uncompressed MFSL and direct-to-disc recordings, which has a way bigger dynamic range than most the crap available on CD. I cannot say what the peaks go to (probably 80-90db I guess) but the average is around 60db and I get the complete sound picture.
A truly good audio amplifier can create a complete musical picture at low SP amplification levels without losing the lowest levels of details. I have heard tens of systems where they needed to be turned-up-to-sound -right. Without exception, at low volumes they lack the small nuances and micro details that natural music has. These system simply have to be cranked because they suck at low volumes.
Regards
Paul