Spry and in top form, it programs a mixed bag of sturdy plays, conversational touchstones, edgy musicals (The Scottsboro Boys), and affordable opera, then stands back and hopes for frisson. It often achieves it, and if it fails, it doesn’t dally long, since it has three theaters (seating 500, 160, and 80) to fill. There’s nearly always something high-quality to plumb here.