I have a Kubota
B7800 with loaded rear tires ("beet juice"). I have encountered no balance or other problems when my subframe mounted BH is attached. Because the BH weighs considerably more than any of my 3ph attachments other than the
chipper/shredder, I think that the loaded tires are very important when you are using your FEL without the BH attached, and certainly if you are going to attach FEL pallet forks to it in order to carry heavy loads on the FEL.
If you analize the physics of it, they certainly don't aggrevate any "front light" condition even if your not using an FEL. If you have enough weight in the rear to lift the front of the tractor, it is going to pivot upwards on the rear tires. Since the rear tires themselves, where they contact the ground, become the pivot point, the loading in the tires on both sides of their contact point with the ground cancel out and the weight of the tires, whether filled or not, has no affect on the front lifting.
What loaded tires obviously DO affect, is the tendancy of a heavy load in your FEL to pivot the tractor on the front tires lifting the rear off the ground. Loaded rear tires and whatever attachment you have on the rear end, serve as the ballast that prevents this.
As far as performance on soft ground goes, my tractor with BH and FEL weighs in the neighborhood of 4000#. I doubt that the rear tires, which probably add 300#-500# at most, would have much affect on whether you are going to sink in, although in soft ground a tractor with loaded tires may do a bit more damage than without. However, since you are only adding about 10% to the weight of the rig, I doubt that there would be very much difference in the damage you do.
All in all, I think that the benefits of loaded rear tires far exceed any disadvantages, unless you are planning operate your tractor over your lawn frequently and in wet conditions. In that case, you might end up doing a bit more damage with loaded tires. Since you aren't planning on doing lawn work, the advanages of loaded tires are decisive.