Krell S1200 -used - price differences


I'm in the market for a used Krell S1200.
I have noticed that they tend to be listed in two price ranges - about $4k-$5k, and then $9k+
I can't really seem to find any differences in the features that would explain a doubling in the price for one versus the other....any ideas?
Thanks
Randy
babyseaotter99