Experience with adding super tweeter?

@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. 

"If I run that test blinded very few people over 50 would be able to tell the difference." I doubt that very much. If frequencies above say 12 kHz are missing, my money is on making a clear difference and high blind test colleration for 50+ audiophiles. 

With respect to speaker capabilities: a 12" driver can also 'get to 20 kHz'. The only question is HOW. There is a reason why some of your cited 'super systems' use beryllium for the material of their tweeter. Why not cast iron?

I am NOT claiming at all that super tweeters are needed. Instead, I was wondering WHAT they do when used either as 'add on' in addition to existing tweeter, or for example in a dedicated additional pass (band pass for existing tweeter, high pass for the super tweeter). Someting to consider for example to 'make up' for overdamped rooms?