Regarding the 600 heads and clutch issues. Deere has made several changes to the clutch on these heads, trying to help the issue however after talking with techs where the clutches are made the units are designed correctly and work correctly when used in like conditions. in other words if you stuff too much grain in the head and happen to pinch enough under the edges of the flighting against the trough, the clutch will do its job. However the clutch will not work when a rock enters the head. The 900series heads had shorter flighting which would not allow a rock to get between the tube and floor, granted Im talking of the kind of rocks that will destroy these heads, and Ive done it. What get the 600s is when a rock maybe the size of a football or volleyball gets under the tube and conveys over to the center area it kindof lays there long enough to tear off the inspection door and on the next revolution gets hung up on the hole, wedged against the floor and bends the auger. The clutch isnt even close to having its limits tested at this point, doesnt take much force to push the tube up enough to bend it.
There are ways to change the torque limits of these clutches measured in newton meters but you would have to drop holding capacity soo far for the rock issues that you couldnt get a good feed of crop into a hungry machine.
THe 600 heads are still by far a nicer head, better knife drive, sickle is better, guards last twice as long, feeds more, etc but a guy needs to build rock dams just behind the guards and raise the reel up a bit if he needs to run in rocky conditions.
As for our rocky conditions, I held on to my old 930 which still does the job.
Hope this helps.