Well gee Eric, I'd say if you can't lug her down you're just running it in to low a gear!LOLIt more or less comes down to trial and error and find out which was works best for you to grade out dirt. As a few have mentioned you need to finess the BB up and down to get an even spread. A 30hp tractor with a 6ft BB will do anything you want it to do. Its often times said with a Kubota you'll loose traction LONG before you loose hose power. So a little bit of a bog down is normal is any situation, after all these are tractors and not bulldozers. If you want to drag a box and not bog down get a 100hp tractor and a 6ft box, then I'm almost certaint you'll never bog down.
Ed