The 2k's key feel is it's main weakness. I'm ok with it, you may not be. The 2k is a hell of a lot of keyboard for the money and I would be surprised if the kn2600 packs the same bangs-per-buck.

I thing a good secondhand KN6000 is too close to call and will be down to personal
preference. I've got a 2k but when I got it I was very tempted by the KN6000. Its a bit bigger and heavier but then again built like a tank so you cant have everything. Did some gigs where the "organist" used a KN6000 and I was very impressed by the sound quality. You can fit a hard disc to the KN6000 but I don't know what drives are compatible and I do know the Technics drive used to be very expensive.

I concluded that a New 6k was just too expensive, and that I could survive with swapping floppy discs on-the-fly (you can swap floppies whilst a midi file is playing on the 2k) if necessary.

Other members of this forum know lots more about the 6000.