I just got an M-B4S P from Kanstul. I ordered it with a #16 throat and the Prana backbore and I'm glad I did. I've been looking for a MP that plays this way for a long time.
My goal was a MP with a rim and cup similar to the Warburton 4SV, but with a much bigger throat than Warburton makes. I've been using a #12 backbore and have opened up another #12 to a #16 throat, but my control with the altered backbore was a little rough.
The M-B4s / Prana has the easy high register of the 4SV with a huge sound. The tone with my Kanstul 1500S is exactly what I've been hearing in my head, but couldn't produce.
Workmanship is great; just what you'd expect from Kanstul.
I'll gold plate the rim and cup tomorrow.
You gotta try one of these!
John
http://home.cfl.rr.com/themuterack/
Cape Canaveral