FWIW, I did some adjustments after a few hours of listening: I currently think that the HPF set at 65 Hz works best. (I chose 65 Hz because that is the lowest note the cello plays.) I presently think that the subwoofers do interfere with solo piano music, and so will probably opt to turn the subwoofers off for solo instrumental music, and possibly duets also.
The subwoofers really seem add the most to any complex, orchestral music, as everything seems to become much clearer, more crisp, detailed, and dynamic. In fact, overall SQ must have something to do with more than just the theoretical frequency of instruments, as I found that while integrating the subwoofers, all instruments, including the human voice, became much clearer, and I was hearing instruments that I hadn’t heard before, which were in the upper frequency ranges.