Frequencies >20kHz?

If a tweeter is really reaching it's physical limits at 20kHz it's probably got a resonance well within the audible range. If you can get it to reach up to 30 or 40 kHz that resonance is probably never going to reveal itself. You see a lot of metal domes go that route since their resonance is sharp. Soft tweeter, not so obnoxious. 

Alas, as most of us grow older our hearing ability declines. 15k is about the upper limit for the 50 + audiophile!

Hi bf.

Welcome to the forum. Yes, this is the "right" category. Not to worry if it isn’t though, because there are so many miscategorized treads throughout that categorization has little relevance here, IMO.

I believe ultrasound does play a role in a tweeters ability to accurately produce 20Khz. And it’s for this reason many tweeters go way over 20Khz (to ranges like 35Khz. etc.). IMO this is analogous to automobiles in that while the speed limit may be 55, 70 or whatever mph, in order to have the vehicle move that fast comfortably and reliably, it needs to be capable of 120 or so. A tweeter is no different. If it’s rated to go "only" to 20KHz, it likely cannot (IMO, accurately and with no distortion) achieve 20KHz.

To your point about not "feeling" it, I would say this is perfectly normal. Bass frequencies are "felt", high frequencies are not.