I like the extension of a Tannoy ST-100 super-tweeter on top of an Esotar 330D tweeter that has good HF extension of itself. (In the Merlin application the Esotar is down several db at 20khz.) But while the ST is set high at a nominal 18khz, in actuality its third-order crossover rolls down into the more audible mid-treble range. It’s conceivable that a ST is a glorified hearing aid for aging audiophiles with attenuation in their own HF response.
The ST does open up perceived treble extension and air. But put your ear close to it and all you’ll hear is faint chirping.