I don't know what type of material you will encounter, but I knew from the start that I had a lot of substantial roots near the surface. As other people will tell you, weight is important but when hitting stubborn objects (roots, stumps, rocks, etc) it's even more important that the box blade have a stout construction. I bought my blade with 3/8" steel side panels that will stop the tractor before they will bend. I learned in this forum that a bent BB has no more value.
My tractor is an L3130HST that I run in low range on the first pass with the rippers down and the top link as short as I can turn it. When encountering an object stubborn enough to spin the tractor's wheels, I keep power to the wheels and gradually raise the three point hitch. Usually a 4" root or a 6" stump will pop to the surface. The blade is 6' wide to ensure coverage of the full tractor width, and I have run the box full of dirt even in the medium range. Low range is used with the rippers to keep my speed down more than to get more power to the ground.
Good luck!