I own a 2520 with and FEL and for the most part agree with folks that a loaded ballast box and filled tires or wheel weights are the way to go... even says so in the Deere manual.
However, in my application, I wanted the tractor as light as possible to cut the fine fescue grass on my delicate lawn without leaving tread marks (R4s) or compressing the dirt.
So I elected to add an extension to the ballast box so I could load it to near 1,000 pounds. With this setup, non-filled tires and no wheel weights, I have been unable to lift the rear wheels off the ground with the heaviest of FEL loads. In fact, I'll lose hydraulic lifting capacity (lever full aft) well before the rear-end even begins to budge.
For cutting grass, I remove the FEL and ballast box and voila' light tractor. Obviously your mileage will vary.