I agree that the "factory" presets are not very good, even on the specialized packs like Ruckers. However, with meddling in Pro, I get a reasonably convincing harpsichord sound. Much of the difficulty is that the PTQ factory presets are built much more like pianos, so the default settings are not nearly what you'd find on a harpsichord in the wild. In my experience, harpsichords are somewhat naturally out of tune and somewhat lacking action regulation when compared to a piano (as far as modern piano technician ears are concerned), even when they're freshly tuned, so adjusting the presets to values that would be considered "bad" for a piano are actually good for an authentic harpsichord sound. The default presets are too mathematically correct to my ears, which is what makes them sound bad--at least to me. I've never encountered a "perfect" harpsichord in real life (as that's even more impossible than a "perfect" piano and if there were such an object I wouldn't want to play it: it's the personality and flaws of an instrument that give it the character that spawns creativity and emotion, without its "flaws" it feels and sounds dead to play).
I start by placing the microphones underneath the soundboard (as is typical of most best practices for most harpsichord recording technicians). I place one in the center of the back and one roughly below the center of gravity (I like the C414 and M49 Figure 8 for this), both pointed upward. Then I place a room microphone comically far away from the instrument and turn off delay compensation (you want the phase problems to add character not to mathematically avoid them--though phase inversion of the room mic may be necessary, I just move it far enough away that it goes back into phase--almost), typically selecting a C414 or M49 omni. Then I turn the volume down to -9db or so, take off the Limiter. I also radically changed the reverb to a very long tail but dry balance, usually with a Quad IR or external algorithmic reverb (like a Bricasti or old Lexicon--plug-ins usually because we all work with what we've got). Feel free to mess with the mixing of the main microphone pair from a stereo standpoint, just go with what sounds good after experimenting.
Then I start removed the "piano" mentality from the preset. I go to well temperament or another non-equal temperament (why on earth do we use arbitrary 415 tuning in equal temperament, if you want equal, go with a Baroque equal tuning method that was available not a mathematical philosophy...tsk, tsk, tsk, tsk, tsk), and I detune generously while making sure that the unison width is not somehow enabled (seriously, on a harpsichord!? in fairness, some of the multi manual presets probably use unison width as a detune between individual string sets but I still find it odd). Then I heavily randomize the damper position under the action heading (as it's an additional setting to the plectrum behavior randomization available under "Voicing" in Standard and Pro) generously as well. I increase the key noise and enable its humanization option. I also heavily randomize any plectrum settings, particularly those related to timing and inertia. I will sometimes mess with the string length parameter in Note Edit to try to simulate inconsistency between string mass, density and manufacture across the keyboard compass.
After that, I'll up the "wear" slider to something between (roughly) 0.45 and 0.75, while also changing the seed parameter until I like it (which makes a huge difference). Afterwards, there's a hidden stretch points option (at least on Ruckers) that allows you to adjust special mechanics of temperament, stringing, and tuning when you Note Edit the "Direct Sound Duration" in addition to the other direct tuning, temperament, etc. settings available under Note Edit. Of possible use (though extremely complex and difficult to use) is the Spectrum Profile which allows note-by-note overtone editing parameters.
After changing enough of these profiles, you will end up with at least a Harpsichord-like sound that's more convincing than the out-of-the-box factory sound. I just did a Buxtehude manuals-friendly fugue, which sounded quite convincing to my ears on the Ruckers with the above changes to its L8+U8 factory preset. That said, I'm only more of an organist by trade than a proper harpsichordist; so, like others, I'd defer to a better harpsichord scholar's opinion on exactly what parameters to change to get a "perfect" harpsichord sound.
Spotify:
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Casio GP300 & Custom organ console
