Wow, seeing this thread pop up is a trip down memory lane.
Eighteen years ago I wrote the first reply of this thread, and then-unnamed prototype speaker I mentioned was the magnificent Summa by Earl Geddes. Earl ended up selling his designs direct and mostly as kits, and has since retired. And about a year after that 2005 post I became a speaker manufacturer too, with my designs all drawing on things I learned from Earl.
Duke