Hearing Damage over 20khz?

There is no evidence that the ear detects wavefront slope. The ear is like a digital device - hairs in the cochlea move and trigger nerve bundles. The on and off of the nerve bundles allows us to determine frequencies or tones. The combination of frequencies and the way they decay determines the timbre or sound.

but there are subharmonics generated below these supersonic frequencies that translate into harmonic information and/or spatial cues in the range that we DO hear.

Subharmonics in a bell is called music.

Subharmonics from an audio system is simply unwanted added distortion!

If you add a device that is only supposed to make sounds above 20KHz (and nothing else) and then you hear a difference then you must necessarily be hearing distortion from said device.

If there is audible sound between 20 and 20 KHz and it is on the recording then you will hear it - irrespective of what source (instrument) made these sounds and irrespective of whether some of these audible sounds are subharmonics of higher frequency fundamentals of certain instruments (possibly percussion?).

The whole thing about sound reproduction above 20KHz is misguided at best - at worst it is deceiving.

The only valid argument for frequencies beyond hearing capabilities is that better performance beyond human hearing capability might mean the system performs more accurately and linearly within the bandwidth of human hearing. (although this may be doubtful as resonances outside the audible bandwidth may equally be a source of distortion)

What is your explanation of ability to sense the existance or absence of frequencies higher than measured sine wave sensitivity?

Huh? Not sure I understand you. I don't think there is evidence that we hear higher frequencies.

A square wave is made up of all odd harmonics...you hear the odd harmonics up to the point where you can no longer hear them (around 15 KHz for most people)...so you do not hear the "slope" of the square wave but within your audible hearing range you hear the harmonic content that makes up the square wave.

Yes there is some interest in frequencies higher than "audible" however it appears to be the non-linear properties of the air and the way differences between high frequency tones that can make audible sounds (beat tones).

Fundamentally what we hear is still within the audible range of frequencies and there is no evidence that we can actually hear pure tones well above 20 KHz.