Martin

I think you make some fair points but a number of things still bother me.

Recordings made before 1990 don't have any content above 20Hz. I doubt they would go that high, 14-15Hz possibly anything else is probably harmonics.
HiRez 24/96 is not necessarily about hearing above the limit of human hearing, I think its about detail within the frequencies most of us can hear and that's a lot less than 20Hz.
Super Tweeters can produce sound way above the level of human hearing, we can't hear the real frequency but we are able to hear the harmonics of the higher frequencies. To extrapolate from that, 24/96 and higher resolutions can be heard due to these same harmonics.
I would conclude that HiRez does have an advantage even to those old analogue recording because we are not only listening to what was recorded but also the harmonics of that original recording.
CD cannot and does not match a well recorded analogue source as it is does not have the bandwidth to reproduce all the harmonics. Nothing to do with frequency extension.