http://www.jeroenbreebaart.com/audio_vst.htm

Try loading one of the recent S.Erard fxp's and then, in this saturator vst, use either the preset How About This or the preset Analogue Circuitry Sim.

At the bottom of the interface, turn the saturation up to around 4 %, give or take a percent. TURN DOWN YOUR VOLUME SLIGHTLY.

I know that it seems strange, if not simply wrong, to apply an effect that degrades the sound to that of a recording. Consider this as just a way of getting another sound from Pianoteq. Also worth trying, more for a general timbrel change than a change in the bass, is reducing the tape speed. Putting it past the midpoint causes notes to repeat, but in the range of 80% and above, it changes the harmonics quite a bit. Generally speaking, the lower the tape speed, the higher you can raise the Saturation. But be careful about feedback. (One thing that's nice if you use Cantabile\Cantabile Lite, or a similar program: you can attach these controls to midi cc's, and thus have new timbrel controls--do try varying these settings along with the parameters in PianoTeq.  The saturation and tape speed changes are not needed, but they do offer a very different take on the sound each time you change a PianoTeq parameter.)

AND: The midrange may not always sound good with this saturator\tape emulator, but try playing fairly hard in the middle of the keyboard. Hit a few low notes both at ppp and at ff--a fat but precise, close sounding bass. The upper treble is interesting too--there seems to be less variation in the timbre from soft to hard strikes, but the sound is its own and worth having as a variation.