Interesting read. According to the article linked the tweeter built in the 1950's was unable to reproduce frequencies below ~5khz without significant distortion. A conventional McIntosh tweeter was far cleaner than the plasma tweeter. I imagine significant improvements have been in the last 50 years, but I wonder if they're still experiencing trouble creating the lowest frequencies the tweeter handles.
The marketing PDF for the Lansche notes the smoothness of the tweeter well beyond 30khz and makes many assertions to its lack of distortion at all frequencies. I'd like to see some modern tests of the Corona, but I'd also like to just listen to it.