Yamaha Be drivers are pretty much unequaled. They have extremely low distortion. After I bet heavily on them in 1976 I was very pleased to see Martin Collums mention them in his book "High Performance Loudspeakers" which was sort of a predecessor to Dr. Toole's book. In it he said that the Yamaha Be drivers were "the lowest distortion tweeters (with the same notation in the mid range chapter separately) that he had ever measured"
or words to that effect.
Then almost fifty years later on Page 453 of Dr. Toole's book one can't help but notice that on that page which shows spinoramas of various vintage speakers, the only one that really yields outstanding performance by todays standards is the NS 1000.
I think Yamaha knows how to build superb loudspeakers. Before I would buy the rather pricey NS 5000 I would try and find a pair of NS 2000 which are reputed to have even lower distortion than the NS 1000. There is a pair on fleabay right now for $5K the pair.
The Yamahas are the only Be drivers made by vapor deposition. The diaphragms weigh essentially nothing.(tweeters = 30mg) Much less than any subsequent attempts including Focals and TAD compression drivers and a few others all of which are formed from Beryllium sheet metal. They are likely superior to most other diaphragm materials but not up the the Yamaha originals.
Also people should not be confused by high priced polycrystalline diamond tweeters. These are built as an extra revenue source for companies making diamond electronic substrates and should actually be very cheap and perhaps when TangBand decides to build one they will be. There are inexpensive polycrystalline sapphire tweeters that probably perform better than diamond since the density is less and the stiffness close. In ten years Parts Express will have cheapo diamond diaphragm tweeters.
https://www.parts-express.com/cat/tweeters/17?N=19813+4294967118+4294960672&Ne=10166&Nrs=collection()/record[endeca:matches(.,"P_PortalID","1")+and+endeca:matches(.,"P_Searchable","1")]&PortalID=1
https://www.parts-express.com/pedocs/specs/264-865--tang-band-25-1719s-spec-sheet.pdf
If I were building some DIY spikkrs today I would probably start with the above tweeters.