But then again the "average" human can't hear anything above about 17kHz anyway.
Sony has a new line of speakers out where the tweaters are what they called to be "Nano-fine" tuned. They respond to sound reproduced up to 50kHz. I just bought their $279.00 pair of floor speakers which are outfitted with dual 8" woofers, one 3" midrange and one 1" tweater. They sound magnificent.