I think when you here a keyboard and you realise it is a keyboard then there is something wrong.
When I heard people play the technics kn6000/6500 you couldn't guess it was a keyboard or a band.
That's the main point for voice quality...I don't talk about other qualities off the keyboard.
The only thing I am personally interested in is voice quality.

I also had a solton sd1 and the sound was very good indead but I found it very difficult to manipulate...maybe I'm not so clever...so I sold the Sd1.

Alain