Experience with adding super tweeter?

@kraftwerkturbo I helped set up a pair of Hill Plasmatronics. They never show you pictures of the nitrogen tanks out back and they were not particularly well made. They were ugly as hell and most of what you were listening to was regular dynamic drivers and crossovers. Only the tweeter was plasma and they were worse than impractical. 

Most old people can't hear a thing over 12 kHz or even lower. It is called presbycusis. If a speaker requires a super tweeter it is a bad speaker. The "air" frequencies are in and around 6 kHz. Adding more "air" might seem pleasant, but it is distortion. You do not hear all that "air" at live concerts. 

The best tweeter made is the Magnepan ribbon tweeter. Magneplanar will not sell them separately. To get one you have to have the serial number of two 20.7s and there is a core charge. You have to send the old one back. A ribbon tweeter has to be at least 5 feet tall to be practical. Line sources beam vertically. You can not hear a thing but reflected sound above or below them which is why major speaker companies will not use them. At 5 feet you can cover both the seated and standing positions. Short ribbons have to be exactly at ear height.  

@mijostyn Even if we cannot 'hear' (correct: hear less) above 12k (or 16k for that matter), it is nevertheless a key part in any music. Simple test (already mentioned earlier): borrow a low fi graphic equalizer, push all levers UP, and the last one DOWN. Observe. Even an 80 year ("can't hear anythign above 8k"?) will immidately notice the difference. Or take a nice recoreding of a stradivari, and run it through a 12 db low pass digital filter with 16kHz. Those overtones (you call them distortion, the Stradivari owners will strongly disagree, as do most audiophiles) are actually key and truly 'make the difference'. "Adding" a supertweeter may or may not be the way to go; but relieving a heavy, big, lazy tweeter from the 'heavy lifting' above 10kHz for example is just as good and idea (potentially) and adding an 12inch driver to make life easier for that tiny 6" in your 2 way speaker. 

When I started a thread on this topic, I got posts telling me that it is not possible for me to hear high frequencies and it's just impossible for the supertweeter to make a sonic difference. Ignore the naysayers if they chime in.

There are many clueless know it alls on forums. There are doppler patterns @ supersonic frquencies which will produce frequencies well inside the audible band. 

ALL the Vandy carbon fiber tweeters are pistonic out to 30 k… and correctly time aligned and in phase w the other drivers….. excellent systems engineering and sonics…

@kraftwerkturbo I'm afraid that is lay intuition kraftwork. If I run that test blinded very few people over 50 would be able to tell the difference. 

Good tweeters have no problem getting to 20 kHz. Many of them go higher. Of all the super systems I have heard, maybe 30 of them, not a single one had a super tweeter. Atma Sphere MA 2s have a wonderful reputation driving Sound Labs speakers. It is laughable because due to the amp's high output impedance and the super low impedance of ESLs at high frequencies the Sound Labs roll off at 6 dB/octave at 12 kHz and the people with this system never noticed. They think it is a great combo. It is below 12 kHz.