I'm no good at characterizing the subtleties of different tweeter materials or types, but to my ears, the 8351bs produce the most natural sounding treble I've heard. I don't own any ribbon tweeters, but I definitely prefer my Genelec's treble over the treble in my Emotiva T2+, which uses an AMT. But yeah, as far as materials go, I don't even know what the tweeters in any of my own speakers are made of
.
I've thought about buying
@Dennis Murphy 's BMR many times, but never pulled the trigger. His future towers also interest me as one of the cheapest *almost full range 2.0 systems you can build.