Custodian I asked the same question a few weeks back-do a search on Super Tweeters in the forums you'll find some good information.
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?