Talk to orchestra execs around the U.S. about their biggest headaches, and slumping summer sales will likely make the list, even for big-budget bands like the Philadelphia Orchestra and the Chicago Symphony. But in Minneapolis, where the Minnesota Orchestra has been mounting a major summer festival in the heart of the city for more than a quarter-century, ticket sales have actually risen in each of the last three years. Even so, the orchestra has been playing things awfully safe in recent summers, programming mainly crowd-pleasers and avoiding anything the marketing department might find unpleasant. Striking the right balance seems somehow to be more difficult in warm weather than cold…