KBIA's Trevor Harris always comes up an interesting angle that makes me think A LOT after every interview.
*Disclaimer* There are many other outdoor venues, of course (Chautauqua, Edmonton, NYC Central Park) but the three places I mentioned (Aspen, Ravinia and Tanglewood) I was also a participant over various summers.
Check out this article with quotes also by Chris Campbell (Executive Director, Missouri Symphony Society) and Dr. Rob Shay (Director, MU School of Music). http://kbia.org/post/can-classical-music-work-outdoors