Steve, I have run many pairs on the 990. For you I would think the rear tire would be just fine. The only thought is the front will definitely have less bite in the loose gravel and sand. That is what I had on that day we rode Panell rd and it was muddy. Hence the little bit of napping I did in one of those big mud channels. I would try one on the back first. If you like and find a good deal on a front you can try that later. I would recommend a 606 or TKC front if you find them cheap since you like the dirt a little more and don't usually get to stupid on the pavement.
As I recall that day in the mud is the only time I ever saw you open that 950 up and play. You may like the fact that the back will break loose a little sooner on the loose gravel.