In Basel, that autumn of 1937, the Busch Chamber Players brought a Bach cantata and works by their leader's other professed favourite, Mozart, into their repertoire. Now that Mozart's music has become so accessible, it is surprising to discover how little of it was performed before the 1941 sesquicentenary of his death stimulated his rise in popularity. In the interwar years, even the Salzburg, Würzburg and Munich festivals did not probe far beyond the handful of familiar Köchel numbers. In 1917 Busoni complained to Volkmar Andreae: ‘Mozart is altogether terra incognita; for how much is known and played of all his 650 works? While not a single bar of Dr Johannes, for example, is overlooked, whether for clarinet or – contrabassoon!’ When Schnabel performed three of the piano concertos in Vienna in 1934, k503 in C major had not been heard in the city since the composer's time. When Serkin made his United States orchestral debut in 1936, the critic Lawrence Gilman described k595 in B flat as ‘a forgotten piano concerto’. And in 1935 Fritz Busch wrote to Adolf: ‘Do you know a Mozart quartet for flute, violin, viola and cello in D major? It is something especially beautiful’. He had clearly come across k285 for the first time. Even the land of Mozart's birth was slow to appreciate him: in the mid-1980s the Viennese essayist Hans Weigel, born in 1908, wrote:

In Austria everything happens belatedly, at second instance. Thirty years ago Franz Schubert barely existed, Mozart was only taking on his true dimensions when I was already in control of my conscious mind.

No wonder there was so little understanding of Mozart's style, let alone the difference between his and Haydn’s. Yet through the 1930s a quiet Mozartian revolution was going on, in which the Busch brothers played their parts. At Glyndebourne Fritz brought Così fan tutte to the fore, as he later did with Idomeneo; and although he was rightly criticised for omitting appoggiature from both recitatives and arias, he did offer an alternative to Furtwängler, who romanticised Mozart's scores, Beecham, who prettified them, or Walter, who treated them too lovingly. ‘Walter was inclined to over-sweeten Mozart and Busch reacted against that’, said Berthold Goldschmidt.