Hearing above 15kHz

I feel your pain. I too don't really hear much past 15K.

That specific tweeter may not be as useful to you as it would be for a younger person, but all of the speakers you mention have their own "sound" that includes the rest of the spectrum. When you listen to these speakers you are either going to have a preference for them over other speakers or not.

Every product you buy, whether audio or otherwise, probably has a feature or two that are somewhat superfluous for your needs. That shouldn't keep you from buying it if the other performance aspects are perfect for your needs.

Correct. There seems to be little evidence that tweeters that go accurately up to 30 KHz and more are actually necessary. In the end all you gain are bragging rights. However, a better tweeter response within the audible range (up to 20 Khz) will of course provide benefits. (There is a lot more to a better response than just frequency range, however simple specifications lend themselves to marketing hype)

Although you can't hear a pure sine wave test tone above 15KHz you probably can sense 15 KHz roll off. A 10 KHz signal (for example) which is not a sine wave may have a wavefront steeper than a 15KHz sine wave. Based on my own (68 yr) ears I think that the hearing sense involves steepness of wavefront. I can think of no other explanation for what I hear.

My hf hearing at 50 years is long since shot due to working on oil platforms for years before health & safety ruled.
My speakers do 40kHz and I can still enjoy ( notice not saying hear) the benefits of super & ultra tweeters.
So who knows?

Good point Mlsstl. I cound probably disconnect the ribbon tweeter in the Helicon and not notice any difference. But if the balance of the speaker sounds great, that's all that really matters.

Perhaps, as mentioned above, we can sense quick rise-time or high intensity sounds at or above the fixed pure sinusoidal limit of our hearing.

I see an experiment unfolding!

I am a few years over my half century. I tried out Townshend Supertweeters and could'nt hear the slightest difference, except an unpleasant screech with my ear against the supertweeter at maximum volume. Luckily, Max Townshend is a gentleman and took them back with a full refund

Although you can't hear a pure sine wave test tone above 15KHz you probably can sense 15 KHz roll off. A 10 KHz signal (for example) which is not a sine wave may have a wavefront steeper than a 15KHz sine wave. Based on my own (68 yr) ears I think that the hearing sense involves steepness of wavefront. I can think of no other explanation for what I hear

A steeper wavefront than a 10 KHz sine wave indeed means there is a much higher frequency signal there. In fact stereo music is two channels of a single but complex sinusoidal waveform containing a great many frequencies (if looked at over a sufficient time period). Fourier Tranforms are a method to extract energy content by frequency.

Your observation does not imply that you necessarily hear these much higher frequencies (unusual but we know dogs do - so it may be possible).

The only explanation I have is that you may be hearing some form of intermodulation distortion (difference tones) in the ear. It is well known that this happens at audible frequencies, however, I have not seen research to indicate that it conclusively does not occur above audibilty. For example, normally people can't actually hear a 25 KHz tone or a 21 KHz tone but might some people actually hear the 4 Khz difference tone between them? A crazy thought!

It seems unlikely that difference tones might be used in musical instruments above audibility....however the non-linearities and intermodulation distortion in the ear have been studied extensively and does exist.

Intermodulation in Hearing

Might a shimmering Sabian cymbal produce very high frequencies that the ear can perceive at lower frequencies through intermodulation distortion? I suspect not, as the industry would jump to publish many papers and sell a whole bunch more equipment if they could conclusively prove this sort of thing....and I have seen nothing so far. And man has had thousands of years to play around with different forms of musical instrument....so surely someone would have discovered it!

Nevertheless you may be on to something!