“Last night [we] saw a film as part of the Berlin and Beyond Film Festival at the Castro, called Mein Name ist Bach, about an encounter between Bach and Frederick of Prussia. So much of it was fictional, I'm not sure what we could count on, but a son of Johann's, Wilhelm Friedeman, played some of his own compositions, which sounded like they would have been written a hundred years or more later. Do you know anything about his work? Seemed kind of wild and dramatic.”
This is what is called “Sturm und Drang”. The most important thing to understand here is that J. S. Bach lived on well past his era. By 1740, 10 years before Bach’s death, no one was writing counterpoint any more and Bach was the master of a dead art. Thus Art of the Fugue; in this work he was describing his art to future generations. The fugue was basically dead. Churches had entirely abandoned the contrapuntal art of the past. It was a lot like now, actually. Have you been to a Catholic Church lately? It’s not anything like it was in my youth.
Bach was never considered prominent during his lifetime. His son C.P.E. Bach was far more famous. Wilhelm Friedeman was another of Bach’s sons, the oldest, I think. They were doing something completely different. They worked in the court of Friedrich der Grosse in Berlin, and the elder Bach is known to have visited his son there. Whether or not he met Friedrich, I don’t know. Maybe, maybe not. He is supposed to have seen his first piano in Berlin.
The sons played pianos, not organs or harpsichords, and were fascinated with writing things that showed off what you could do with a piano. It sounded more like recitative than anything else, with lots of tempo and dynamic changes: storm and stress.
Primitive versions of the symphony first appeared then. It was a period of great turmoil in music, with possibly a little continuity from the Italians. Handel became old fashioned, too, and shifted to oratorio.
Baby Mozart is entirely in the new style. I hope this answers your question.