tony sayer wrote:

In a hospital hearing test It might come as a surprise for some to
find that they can be 30 odd dB or more even, down from 400 Hz to
around 8 kHz and then might worry about their amp being within a .1
of a dB from DC to Daylight;!...

You convey the quite popular misunderstanding that hearing threshold is an
expression of frequency response, it is not at all like that. The sense of
hearing is auto-aligning itself to the properties of the transducer - the
ear - as long as the sound in a critical band is above the threshold and has
zero detection if a sound is below. A simple concept, except that the bands
are bandpassfilters with a fairly low Q. That has the consequence that
signal in a nearby band weighs in and helps tip the scale of detectability,
something that explains that people with quite large threshold shifts may
have completely valid impressions of the treble on a recording or at a live
concert.

The auto-aligment feature is what makes it possible to "listen through" the
frequency response variations of loudspeakers and rooms as long as a
suitable "listen in" period is available. What is more important than mere
frequency response is the amount of information, there is no information
loss per se in a frequency response variation, but there is certainly a
grave information loss in case of severe resonances or rattle and buzz,
something that also the can be caused by mechanical malfunction in the
middle ear.

Kind regards

Peter Larsen