Consider how much incomprehensible garbage has been dumped on all of us about just these three composers (Haydn, Mozart, and Bach) and you have some idea of how difficult it is to listen clean and to think clearly.
Take the issue of Bach and the piano. I first came to New York after having spent five years immersed in the organ music of Bach, first at Oberlin, then in Germany, where I studied with Helmut Walcha. In New York in the early 1960's no one talked about Bach and, if they did so, it was not the same Bach that I had been working on. In fact, all music written for the piano was considered only in terms of post-Chopin technique.
At that time people still pooh-poohed--even in print!--the idea that pre-Classical musical usage had anything to do with Beethoven or Mozart and virtually no attention was paid to the essentials of articulation, ornamentation, rhythmic grammar, and so on.
I have only now begun to perform Bach on the piano. I am beginning with the Two-Part Inventions approached, not as over-regimented "Classical" pieces with accentuated barlines and Classical-style meters, but approached rather as Baroque inventions, combining imitative with vocal writing, structured yet improvisatory, as is all great Baroque music, even the most rigorous.