DAVID LANG first heard Wagner’s “Tristan und Isolde” at the San Francisco Opera in 1974, as an undergraduate student and aspiring composer. This was the first opera ticket — standing room — that he had paid for with his own money, and he arrived well prepared, with a copy of the score and a flashlight to study it by.

“It was a really big deal for me,” Mr. Lang, now 55, said recently, sitting on a sofa in his light-flooded SoHo loft while two parakeets called noisily for attention from another room…