Thoughts on adding a Super tweeter .

I agree with Muralman1. I have recentlty tried Super tweeters and I think they overlap the other Speakers tweeter's too much.
I also think it threw off the balance of the mid and low range of the main speakers.
But, I was experiminting with vintage Rat Shack Super tweeters. (5K to 45K hz 96db). I knew the original crossover network 5k was too low, so I took all that out.

I tried changing crossover points all the way up to 18,000 hz, and I also tried first order and 2nd order slopes and it still added too much overlap.

So unless someone will let me try a Murata, I am done with that idea for now.

Tom Port uses Townsend Supertweeters on Legacy Focus speakers. He brought over just one and attached it using duct tape at the inside on my right channel Focus speaker at the level of the existing ribbon tweeter. We listened to vinyl and immediately heard the increased airyness of the recordings. The speakers disappeared with the supertweeter (and I have Hallographs behind the speakers in a 25' X 21' X 12' high room with the speakers 6' from walls). If one worked wonders, two should be fantastic. However, the setting was at the low end at 2 (out of 10) because the Focuses are rather efficient at 96 to 98 db/watt. So, this is the first time I've heard a supertweeter blend so well with rolled off speakers (Focuses have a very pleasant rolled off highs, at least the first model from the 1990s did, maybe the new ones have better ribbon tweeters).

No one super tweeter for all loudspeakers, each super tweeter needs to be carefuly matched in level and crossover so no over lap,phase,time problems or boosted frequincys. The one tweeter that matchs all is just asking for grief maybe you will get lucky and it will match but your taking a chance. So if your thinking of adding one realy look into it and maybe DIY or have a pair custom built than you can also match finish as well as transducer crossover.

Most likely the benefits of adding inaudible content are inaudible.

It is known that sharp filters to remove out of band noise can effect the "in band" sound, however, that seems to be the only convincing argument for greater bandwidth (well beyond what you can hear) in audio reproduction.