To make matters even worse, I've also been offered a used KN 6000.

IMHO--The Kn6000 had problems. It is the one Technics model I would stay clear of, all the others are good. I owned a KN6000, after ten rounds trips to the service center, they finaly replaced it with a KN6500.
I do believe because the KN6500 was made to replace the KN6000. The KN6500 was the only keyboard that did not end in all zeros.
Kn800, 1000, 2000, 3000, 5000, 6000, 7000.
The Kn6500 was without problems.

Only my opinion. John C.