Use of a super tweeter worth the investement ?

@alexberger, +1

chorus, I will give them a try soon, and I let you know for sure, thank you .

roxy54, do you have to search before finding the right placement on top of your main speaker, do you have to move them more in front or back or in line of your main speaker tweeter ?

Good question. My speakers have a horn in the center and a 12" bass driver above and below the horn. I tried it on top of the speaker at first as Townshend recommends, but I suspected that  it would be better to try to align it with the horn, so I built a simple pair of wooden stands that were the height of the horn, and then in an attempt to time align them, I moved them back a bit farther than the distance from the mouth of the horn to the dome of the compression driver in the horn. The result was immediately noticeable.

I think that Townshend and their dealers (one of whom I spoke with) should amend their placement recommendation and encourage users to experiment with placement. On top of the speaker might be good for some if the top of the speaker is at ear level and the tweeter of the main speaker is near to the top, but in my case, the top of the speaker is far above my ear level, and if you're using a ribbon supertweeter like the Townshend, you need to consider that they don't have wide dispersion like dome tweeters, so they should be at or near the level of your ears and pointed toward you. 

I recently added a pair of Elac 4pi's and wow.

Like many have said above, re sound stage opening up, tighter bass, much more recognizable detail in all frequencies. I would not want to take them out and actually added a second pair to an office system. I really do feel the super tweeters just upgraded my main floor standers to punch way above their weight and sound like 5x their cost. They weren't cheap but they did add their value and more, imo.