To add to dv's and Qexl's great observations, when technicians tune Boesendorfers with extended range (and if memory serves on Stuart and Sons technicians use a similar technique as well), the technician wears gloves to protect the piano strings from corrosion and presses the middle of the string to mute half of the string and then they tune the string based on the first harmonic (1 octave higher), which--if they muted half of the string correctly--will leave the fundamental reasonably in tune. While that exact functionality is impossible to directly replicate at the moment in PTQ 7, PTQ Pro offers the ability to edit all of the audible overtones of each note, and I suspect that your preset (as Standard honors Pro overtone curves but disallows viewing or editing them) has an out-of-tune overtone that the developers/testers liked or considered "realistic" but for your use has become distracting. In that case, the only solution would be a new Pro preset with different overtones on the note giving you trouble. If you modify a preset in Pro and export it, Standard will load it, but you still won't be able to edit it further within Standard.
Broadening the discussion, the extra lower notes on the selected extended range pianos are considered below the threshold of "music" or "tone" vs "sound" or "noise". As notes lower, we hear less and less of the fundamental and more of the overtones. If an infrasonic sound has a high enough sound pressure level (SPL), we can feel the oscillating vibrations but not necessarily hear the direct sound, instead we hear the intense overtones or higher-pitched sympathetic vibrations of nearby objects being rattled by the vibration, or we feel the vibration physically and perceive it as sound. The tam-tam is an excellent example of this, as gongs--which a tam tam can easily be mistaken for--are pitched while the tam-tam is an unpitched percussion instrument that provides a generic low, loud noise, and it makes no practical difference if one orchestra's tam tam or bass drum is slightly larger or smaller, whereas tuned timpani need to constantly be tuned by their performers while on stage or hitting the wrong gong during a performance will be highly noticeable.
When trying to actually tune a sound that low, you have to be extremely accurate in your reading of the sound, as a simple error of 1hz is significantly greater at infrasonic levels than at high pitches. At C8/c''''' (the highest note on nearly all pianos), the frequency should generally be around 4186hz (or just around 4.2kHz to give an acoustic sense of how slow that oscillation is compared to the electronic communication signal of 5GHz wi-fi), and an error of 1hz will be off by roughly 0.0023889% (that's especially rough as having an error of exactly 1hz is as arbitrary and difficult as being at precisely 4186.009hz for C8/c''''' and we're not even discussing divergent trichord tuning or exotic pianos with tetrachords or aliquots). An error of 1hz with the low A0/A,, will be 3.636% off, a much higher and audible error. This is even worse at C0/C,, at 6.116% for one extra (or fewer) oscillation per second.
To combat the complexity of apply hertz units in piano tuning--as even fractions of a herz become very important at the low range and beyond useless at the high range, technicians use "cents" where each semitone is divided into 100 units of relative tune, making for 1200 cents per octave regardless of range or temperament. So, our first error at the top of the piano was just over two-thousandths of a cent off, which is fantastic accuracy, while the lower errors were 3-4 cents to over 6 cents, which would be audible--especially in the overtones. When a piano note is really sour, it's 40-60 cents off or more, and anything over 50 cents sounds more like a neighboring note than the actual note. Most people can hear a difference of 5 cents or more, but technicians should have ear training that is much more precise (or at least a good tuning device that can). Minor details can change the whole pitch of the instrument. One hall I worked with tuned the instrument 20 cents sharp (if memory serves) across the whole instrument, so that when they rolled the piano out of the pit and turned on the house lights, the heat in that small recital space from the spot floods would warm the strings enough for the strings to "sag" into tune. There are even cases where performers will ask for extensive tuning errors, like when that small hall had a performer ask that the A0/A,, be 100 cents flat to play as G# (it was an Ab but generally technicians like to express piano enharmonics in sharps), since the concert hall didn't have a Boesendorfer with extended range to play the Cziffra repertoire that was specifically composed for the Boesendorfer.
Extended range was a direct result of Busoni's attempts to transcribe Bach organ works for piano, as he wanted the full 32' pipe range which goes down to 16.35hz, but the prohibitive complexity of further extending the piano range (and the complexity of tuning and maintenance) meant the innovation has remained sidelined by all but two manufacturer's of pianos (Stuart and Boesendorfer). With electric pianos, then digital pianos, then virtual pianos, we've gotten to the point that we can functionally emulate that without expending copious resources, but the downsides and intricacies of why piano makers have deliberately avoided those notes for a century and a half still seem to appear in their virtual counterparts. (Don't even get me started on the disorientation concertizing pianists get when they're suddenly on a longer piano--even with those keys painted black!)
Keep us posted on if you find why exactly this extended range preset isn't working for your needs, and we'll see how we can further help you.
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https://open.spotify.com/artist/2xHiPcCsm29R12HX4eXd4JPianoteq Studio & Organteq
Casio GP300 & Custom organ console
