Custodian, you may want to check out the ion tweeters from Acapella. You can purchase them for use with other speakers. I believe the Freq. responce is 5Khz-40Khz. They are the most open and real sounding tweeters made.
Super Tweeters
There are several manufacturers of add-on supertweeters to extend the speaker response well up into supersonic regions. These are reported to "enhance" the sound, particularly of SACD reproduction. I've seen reviews in the UK hi-fi press. The units I've read about include the Sony, Tannoy and Townshend units but I'm sure there are others. What are members experiences with these devices?