classic guitar to create a bass guitar ?

For much the same reason that the Classical Guitar model can’t do hollowbody jazz guitar (or any other type of electric guitar) in a believable way, you also won’t be able to come up with a wholly convincing electric bass guitar sound — we will need different, specific core models for that (of an electric guitar and electric bass respectively) — but you certainly can create bass sounds with Pianoteq’s Classical Guitar that are very useful. It's not on the level of IK's MODO Bass, but it's more than serviceable. At least, I think so.

To go from Classical Guitar to bass guitar, you can either click the Plus button (bottom left) to extend the note range into the region where the bass lives, but I don’t recommend that approach because you’ll loose all of the Guitar’s performance functionality.
A surprisingly simple (if rather blunt) way to get you immediately down into the bass region while leaving most of the Guitar’s performance abilities intact, is to simply change the tuning from 440Hz to 220Hz as that will drop the audio by an octave.
(One downside of halving the tuning like that, is that the performance sounds, like slide and gliss noises, are also pitched down by an octave, but that’s less of a problem than it might seem. Check the demo below.)

Once you’ve halved the tuning, it’s mostly a matter of preference and taste, I’d say. But, like Key Fumbler also suggested, I would begin with switching the instrument to mono output. Then, in the Effects pane, maybe replace the Delay by an Amp (on which you cut most of the highs and mids), add an Eq3 to remove yet more highs and mids, … and in the third slot, you can add whatever you feel gets the sound closer to where you want it to be. A compressor, for instance, could be a good candidate for that third slot. (I used a Tremolo in the third slot for the example below. Which forced me to use a non-Pianoteq compressor afterwards.)

Parameters on the main GUI to look at: increased Direct Sound Duration, reduced Cutoff, increased Q-factor, raised Plucking Noise … Also check which settings under Plucking Brightness work best for you. They do make a difference. And don’t forget to visit the Action palette where you can decrease the Damper Duration and maybe also increase the Mute value (for a more plucky feel), …

As always with Pianoteq: check and explore everything. Some parameters may not seem to make much of a difference at first, until you discover that they influence the behaviour of another paramter which actually does make a difference.

Below is a short example. I did this very, very quickly — 15 minutes, tops — so you have to allow for an unacceptable amount of roughness. I’m confident that with some more time, things could be improved considerably. (Also: for some reason, my license for the Classical Guitar has disappeared. So this example was done with the instrument in demo mode, hence the mostly-white-notes bass part.)

https://forum.modartt.com/uploads.php?f...est_01.mp3

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these are both so cool.  i would love to see more stuff like this in FXP corner.  and even crazier stuff ...

Thanks for all of your suggestions. I'm going down that rabbit hole for the sake of trying.
On Linux, even with Wine, I can't install Modo Bass, that's why I thought about Pianoteq.