Mozart first heard Stein's revolutionary piano in Vienna, which led to significant changes in keyboard music. The innovative design allowed for greater expressiveness and dynamic control, influencing composers like Mozart, Beethoven, and Schubert. Therefore, the correct answer is B. Vienna.
;
Mozart first heard Stein's revolutionary piano in Vienna, where its innovative design greatly impacted keyboard music. The brightness and expressiveness of Stein's pianos shifted the dynamics of piano composition. This encounter influenced Mozart's own works and style. ;