A courtier, errand boy, master sound technician, and Steinway rep all rolled into one, Stephan Knüpfer has the unenviable task of catering to the whims and ears of international piano divas. Yet as this politely smiling, endlessly patient German tries to please Lang Lang, Alfred Brendel, and other stars, this documentary insinuates that their ideal sounds may be more imaginary than heard. Knüpfer arduously tunes and tweaks dozens of grands, like so many Formula One Ferraris; each new sonic possibility he creates only increases the pianists’ doubts and demands. Their acoustic hair-splitting begins to seem like delusion, as if from the case files of Oliver Sacks. When, to combat one hall’s difficult acoustics, Knüpfer creates elaborate louvered panels to fit atop a Steinway (like a giant cheese grater), it’s imperiously rejected. “Pianists are mostly dissatisfied,” he sighs, but he daren’t criticize his masters by name. Directors Lilian Franck and Robert Cibis fail to plumb their subject’s frustrations or any other insightful biographical details: Knüpfer loves his dog, and his wife makes a mean cheesecake—that’s all we learn. Though he can certainly play the piano and distinguish its every audible color and tone, it’s unclear if he even loves music.