In his website review of "Variations," Todd mentioned a faint background noise. As I explained in my article, this was my laptop fan in the hall, which was being picked up by the right-hand microphone, despite the acoustic baffles around the table it was sitting on. As the spectrum shows, this noise is based on a fundamental around 2.5kHz and its harmonics, all at -90dBFS or lower. You can also see a low-level spectral line at the video-related frequency of 15,750Hz.
All the noise-reduction techniques I tried made the sound of the piano worse, so I decided to leave it alone. The noise should not be audible at appropriate listening levels on loudspeakers, although headphones at high levels wil unmask it, of course.