You could always try a ribbon tweeter but again, you need to know specs, otherwise you'd need to modify the crossover, never mind flange sizes and mounting hole pitches ect..

I know I can hear well over 20khz, according to the online listening test someone posted a while back(Peter?). At 16khz, my hearing 'seemed' to start improving. My name was Rover, in a previous life.

Half of communication is listening. You can't listen with your mouth.