Two revolutionary symphonies were paired in the Boston Symphony Orchestra’s second appearance with Christoph von Dohnányi at the 2011 Tanglewood Festival. The musical seed of Beethoven’s ‘Eroica’ lies in the theme of its variation finale, drawn from his ballet music for The Creatures of Prometheus, and specifically in a contredanse where masters and servants traditionally swapped roles. With his Op.9 Chamber Symphony, Arnold Schoenberg sought to compress all the salient features of a symphony within a taut, single-movement work for 16 musicians, and the genre was never the same again.