Hi Nick G.
Your missing the point. Each new sound engine in a keyboard produces a very unique sound. That's why a PA2X sounds different from a Pa1X even though much of the same sample data has been included in both.
Where with a Yamaha, the engine is always the same. You just get a few new samples with each new model that is release.
It's like listening to the same radio for all your life and getting a new CD from time to time. The tune is different but the sound is the same.
Regards
James