Question: What German composer wrote a letter in 1770 addressing Lucrezia Aguiari's ability to perform in C sporano acuto?
Sentence (from "Beethoven and his contemporaries"): Beethoven had well-known fallings out with his one-time teacher, Joseph Haydn, with the piano virtuoso and composer Johann Nepomuk Hummel, and the German composer Carl Maria von Weber.
Relevant evidence: No