from whatever videos i have seen, the BK 9 is far more superior to S750 or 900.
You might've seen some videos I haven't...
But wait, I DID seen them all!

BK9 is superior, if you need 76 keys.
BK9 is inferior if you need a harmonizer, touch screen and sampler.
The rest (sounds, styles, functions) are rather subjective issues.
But certainly is not FAR superior in any regards. Not to Pa900 that is.