Experience with adding super tweeter?

thanks for the input. What happened to the soundstage (size, precision)? 

Makes sense, more 'airy', less 'precise'. 

For test purposed (proof of concept), would it be sufficient to get my hands on a 'super tweeter' (ribbon, similar) and make a simple 6 db/oct high path (with various crossover points to play with)? 

The PHASE thing: at 10kHz, the wavelength is 3.4 cm. So moving the supertweeter through that entire range is not too difficult, maybe in 10 steps of 3mm or so. 

But as with any OTHER speaker, the PHASE will only be "correct" (in line with other speakers) for one specifc frequency. So I guess speaker manufacturers may try to get the drivers 'into phase' at the crossover frequency? Or simply just ignore the it since it is OFF the whole time except for one frequency?

I remember the plasma. Ingenious (no mass!). But widespread use I think was also hampered by the Ozone (O3) that plasma 'flame' generates. Small doses are healthy, but higher/longer can cause problems (cance) IIRC?

Ozone (O₃) is a colorless to blue gas with a pungent odor. Exposure to ozone may cause headaches, coughing, dry throat, shortness of breath, a heavy feeling in chest,  and fluid in the lungs. Higher levels of exposure can lead to more severe symptoms. Chronic exposure may lead to asthma. Workers may be harmed from exposure to ozone. The level of exposure depends upon the dose, duration, and work being done.

Ozone is used in many industries. It is used for purifying air and drinking water, in industrial waste treatment, oils, bleaching and waxes, and to make other chemicals. Some examples of workers at risk of being exposed to ozone include the following:

• Outdoor workers in areas with high levels of ozone
• Factory workers in paper and pulp mills
• Workers in waste water treatment plants
• Fisheries workers who treat storage water

"not hear high frequencies" is certainly nonsense. Just use a cheapo old fashioned equalizer and pull down the "16k" lever while pusing all others up. Then listen to a violin (or pretty much any other instrumetn/music). 

@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. 

"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?