johnk,
Thank you for the very good list of current horn speaker/driver manufacturers. I recently got to hear a system built with new G.I.P Laboratories drivers and they should be on your list. The system I heard had an 18" field coil woofer in an open baffle arrangement, a WE 555 replica midrange compression driver (the horn was a copy of a YL wooden horn) and a replica WE 597 tweeter. The three field coil drivers were powered by a newly built tungar power supply. This is a fantastic sounding system. What was surprising to me is how good the 597 replica tweeter sounded (I did not expect the tweeter to make that much of a difference). I am looking into this tweeter for my system, and maybe, the 555 compression driver (I currently run WE 713b for the midrange).