Welcome to the forum. You will get many perspectives on the keyboards. All are top notch, but each will have it's reasons for being personal preferences.

IMHO none have the sax, flute sounds of the X1, tho I loved the pan flute in the PSR-9000 demo. The Grand piano in the supersolo set for the X1 will match any of the others you mentioned and probably beat most.

Ballad rhythms abound in the X1, in many cases taking "harder", rhythms, modifying instrumentation, tempo, etc and making good ballad backup. Unfortunately all arrangers seem to be a little weak in the "romantic" styles area (at least PSR-8000, X1, KN-6000, which my wife and I have). However, with tweaking of arrangements the X1 can take on romantic songs.

If you sing, then I suggest you eliminate the KN-6000 as it's vocalizer, to be honest, sucks. If, as I have heard, the PSR-9000 vocalizer excedds the PSR-8000 (Ihave an 8000), then it would be a fine vocalizer as the 8000 is quite decent. However, I have heard where the X1 vocalizer is terrific, but until I receive mine, I can't say for sure. Try and hear or play with all or as many as possible before making your decision.

I, in all honesty, would second what Don said.