What's the best mid-range driver available?

yes, there is a place to get just the driver (sans magnet) it's based out of england.

The Skaaning or Flex Driver (other commercial name) used on the Verity is a thing of beauty. Fast as lighting with the best midbass I have heard. My two cents.

Yes the Skaanings are very nice. The Danish DIYer , Mogens Anderson has a 3 way Skaaning + ATC + Scan-Speak. He had Skaan build a 5 inch with specs of his choice. The only thing he would change is the tweet to a Hiqulaphon. Take a look. http://home13.inet.tele.dk/meil/

I believe Solen in Canada sells ATC (www.solen.ca) just for reference. What I don't get with the R-3's though, is I could swear I've seen design/kits using and R-3, but then crossed to an R-2 or R-1 to finish on up to 20khz ?????(Big Time)????? Unless there's some reflections of the wavelengths in the upper registers given/relative the dimensions of the R-3 that actually warrant one of the smaller Ravens better at the high-frequency extremes and the crossover worth it--I can't think of any reason to do something like that. But can't the R-3 be crossed even lower than the ATC dome-like 500hz with the R-3. If it were to be an active system-the money saved on xover parts and an extra set of amps would be a strong point in favor of a two way R-3 setup.

Yes i found access to the ATC's. Since we're on the subject of "best" midrange, it's good you brought up the Raven 3. This ribbon is one very interesting speaker. If i chose to go single source driver this would be the one. The R-3 does go down to an amazing 500,i had thought it was 1K. Zalatroyn has a kit with the R-3. New York Noise Maker. You can see the xover schematic and a picture. The NYCN has dual Focal 10 inch Kevlar woofers in each cabinet. Quite a load for a tube amp i would guess. They xover at 500, which seems low to me. I would feel 1K would be a better xover point, so as not to stress the R-3. Though the unit does looks like it's built like a tank. The R-3 could offer a seemless image that the ATC + tweeter may not. If i used the R-3's i would go with the new Seas W26. W26 + R-3! What a speaker!!...for under 5K!