Thank you MRC01 for your very detailed feedback.
The mics are placed outside of the stage area.
In absolute terms the distance from the mics to the instruments is not big (about 4 meters from the mics to the center of the piano). But this is not a symphonic hall. It is a small Chamber music hall so those 4 meters can relatively be quite more.
Yes I think it would make sense to say that the pickup point is far enough to produce that high frequency roll off you are describing.
We do not do any mixing, panning or any other mixing table work. The image is produced by the stereo pickup and the placement of the instruments within the stage. It works quite accurately because of the treatment and calibration of the hall (pretty much like it happens with the "not masking effect" when one instrument is playing loud and others soft. The differences in the Beethoven and Mozart are due to the different placement of the musicians on the different recordings sessions.
They usually place their music stand on a place they feel comfortable with and do not move until we finish the recording of the piece. But when they come back to record the next piece they sometimes place themselves in slightly different positions.
I think (mea culpa) I should mark these places on the floor so they always use the same spot throughout the whole album.
The mics here are placed at the same distance than with the Mozart-Beethoven (they are always at the same spot for every recording). So that would explain the high frequency behaviour.
About that "buzz", if it is what I think it is, what you are hearing is the vibration of the capo section of the piano (with is a problem that we had with this instrument at the beginning of its breaking in). It might be interesting to read about this. Here is a link to a pdf file that explains all the details of the instrument, its problems and ways to fix them to obtain a beautiful sounding piano.
https://rennerusa.com/wp-content/uploads/2017/06/Selecting-and-Voicing-the-Renner-Hammer.pdf
On page 9 point 2 it talks about this particular problem that we had with this piano at the beginning.
"Noise in the capo section—This is most noticeable in the first capo section of certain brands of piano. If you hear an objectionable tone, try muting the front duplex with your finger while playing the note. If the objectionable tone goes away while the duplex is muted, then deep needling higher in the shoulder may help. Do not permanently mute this front duplex section, as this will cause a substantial loss of power."
Like I said, the problem was solved prior to recording the Rachmaninoff. So the Rachmaninoff and the upcoming Tchaikovsky should not have any of that.