This is what Verdi should sound like. Idiomatically sung by confident singers, with role-tailored voices that make one sit up and listen. The NTR ZaterdagMatinee concert series at the Concertgebouw, a long-standing institution financed with public broadcasting funds, traditionally opens with an opera. This year it was Simon Boccanegra, Verdi’s sombre study of personal tragedy lurking behind public success.