gustavts, In case I misunderstood you, I will try and guide you on this...Bring up the piano you want to alter..for example "Stereo piano"..now click on the synth button to the left side below the screen...now select edit..now you can see the parameters , you can control with the sliders below the screen..
I would try lowering the cutoff[darkens the piano] and change the resonance after you change the cut off[filter]..raising the resonance will make it more electronic[we do not want it to sound too electronic]..Experiment and see what you come up with...Don't forget to save it as a user tone, before going on to something else..