it might not just be tread wear or psi, it might be the trail conditions you are riding. some tires work better on certain conditions than others. for the hardpack sand and rocks, kenda nevegal on the front/small block 8 on the back is good. i like the conti rubber queens, which have changed their name to trail kings, same tire.

go with the lower psi, if that don't work, perhaps you need different tires.