They were designed in the mid 50's by Paul Klipsch for EV.
They were pretty good (and were efficient as you may expect) for the day, but I don't believe they are really competition for modern stuff.
In 1976 and 1977, went I went on many, many speaker listening binges, my experience regarding tweeters was you had the soft domes which were a bit muted. Then you had the metal domes and horns, some of which had a wonderful air (not available from soft domes of the day), but all of these were a bit shrill/harsh resulting in listening fatigue to my ear. Some (if not all) of the Regency's had a brilliance knob which would help with the fatigue issues (at the loss of the "air").
That is my take, but I had friends who didn't have issues with them!
Happily, the better modern soft domes have the air, and the better modern metal and horn tweets are not fatiguing.