Mozart reached new levels of refinement and finesse with his Prussian Quartets, written near the end of his life. Beethoven, as he did with the symphonic form, expanded the possibilities of the string quartet. It should never be performed in public, he said of his Serioso Quartet, but only for a small circle of connoisseurs. It was too cutting-edge for its time. But today, 250 years after his birth, it is a startling example of his genius.