As far as member base is concerned, metro Atlanta is pretty diverse, I've met many members particularly from Cavaliers, Phantom and Cadets from cities surrounding Atlanta.
The major world class corps are pretty predictable geographically...
Spirit & Crown
There are shows in Alpharetta, the Atlanta regional of course, and also one only 1 1/2 hours west of Atlanta at Jacksonville State University.
I actually attend Kennesaw Mountain HS, where there have been MANY drum corps members in the 8 or so years of the school's existence, including the 2007 members, 4 of which were in Carolina Crown, 3 in Spirit.
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The current new conductor at Phantom Regiment, Koji Mori.