@waedi
Just an idea: have you noticed, that the virtual-keyboard is velocity sensitive? The more you click towards the front, the higher is the velocity. Maybe your physical-keyboard produces lower velocity values.

Thank You, groovy, I think you are just right. After fiddeling around all day long, I finally found there is a something called Key stroke dynamic (in the digipiano). When I moved the knob to 'lighter' the Sound of the PC speakers got stronger.  A pitty, the Sound became also more 'clinky', but I hope this a matter of setting the correct Parameters in Pianoteq.
Anyway, thanks a lot for your response,  guess the Problem is solved.

To clarify, are you trying to listen to Pianoteq through the speakers on the digital keyboard ?

To do that you would need to feed the sound output from Pianoteq ( i.e. from your computer ) to an audio input port on your keyboard ( and your keyboard may not have an audio input port ).  The USB connection will not transmit sound to the keyboard, just MIDI signals.

Likewise if you're trying to listen to the Pianoteq sound through headphones connected to the keyboard then you'll hear nothing unless there's an audio output connection from computer to keyboard audio input.

No, as I mention above, in both cases I listen to the Speakers on the PC

To do that you would need to feed the sound output from Pianoteq ( i.e. from your computer ) to an audio input port on your keyboard ( and your keyboard may not have an audio input port ).  The USB connection will not transmit sound to the keyboard, just MIDI signals.

Likewise if you're trying to listen to the Pianoteq sound through headphones connected to the keyboard then you'll hear nothing unless there's an audio output connection from computer to keyboard audio input.