I have experience with speakers that have integral supertweeters. There is an audible difference with some "dulling" of the high frequencies when you cover them.
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?