If you play two handed "live" like I do (without the arranger, sometimes)and you like to layer nice fat strings and pianos - you'll be sadly disapointed with the 9000. The allotment of polyphony is terrible! The advertised 126 notes are not as efficient as earlier 64 voice instruments. This is because Yamaha used many partials for their "primo" sounds. If you are strictly an "automatic" player, the 9000 is as good as any of the rest, although I prefer the "punch" of the rhythm section in the X1 and the VA7.
I wish I had heard the 6000's piano sound - so I can't comment on it - but I remember the Kn5000 very well and I hated that acoustic piano sample. (personal taste)
All in all - your Kn600 is a more "specialized" market piece and is probably going to hold it's value longer than the Yamaha, so I wouldn't make the trade unless you absolutly LOVE it. Please don't take our advice as the gospel truth - it's VERY subjective, as you know. Play it long - see if the dealer will let you take it home for the weekend to do a fair A/B comparison. I'd hate to see you made a hasty decision that you may regret later.
My vote - keep the Kn6000 for a while - The Yamaha will go down in value soon, and will be plentiful. The 6000's are not as easy to find because the MI store don't usually carry the line. Put on your thinkin' cap, and GOOD LUCK !