Best known for rousing pieces of music like “Ride of the Valkyries,” the composer Richard Wagner might not have a reputation as a hopeless romantic. Still, Wagner is also remembered for secretly composing the symphony “Tribschen Idyll” (later renamed “Siegfried Idyll”) as a present for his wife, Cosima, on her 33rd birthday. On Christmas morning in 1870, Wagner and a 15-piece orchestra quietly assembled on the staircase of his house and woke Cosima by playing the piece, now considered one of his greatest works. Deeply moved, Cosima would later write in her diary, “When I woke up I heard a sound, it grew ever louder, I could no longer imagine myself in a dream, music was sounding, and what music!”