it's a combination of several factors (based on experience with vsl STEINWAY D & of course other synchrons)
01 - velocity curve editing : this one is so related to your keyboard behavior, if all you get is fortes and fortissimos then you have to shape the velocity curve like a downward half circle
02 - dynamic : i find the dynamics on synchron pianos to be a little bit "all-over-the-place", i mean the difference between fortississimos & pianissimos are HUGH (in volume) to the point that i'd say it's a bit unrealistic, but you can turn the dynamic knob down (button left - play window) from 100 to 75 and it'll be much more realistic and pleasant
03 - the piano itself : i think the synchron steinway is a bright instrument (nothing against it - it's my second favorite and of course a great instrument and the sound is identical to the real thing at synchron stage vienna)
but if you want it to sound a little calmer and romantic you can use the mixer EQ and FX section, having less high frequencies and a subtle compression would definitely make it a much softer instrument and if you do all the things that i've mentioned plus this one, it'll sound beautiful, i promise
hope that helps