Back to the point, this is a question I've been confused about as well. The popular opinion here has always been you can't have too much weight in that ballast box. If the 3ph lifts it, don't worry about it. But I've been playing w/ the weight/ballast calculator worksheet from JD website, and that doesn't agree, at least for a 2305. If you play with that worksheet a bit, it's very easy to overload the rear axle. With the loader installed (and the weight they use in calculations for that appears to be the loader + max capacity of load) you can still overload the rear axle w/ a 1k ballast box on an imatch.
I'm not that concerned about the ballast w/ the loader. I have about 480lbs of sand in my box that I use whenever anything's in the loader bucket. But the snowblower is where my questions lie. Given that the blower is about half the weight of the loader, I don't see that I'm that far from overloading the rear axle w/ the blower and my ~580lb ballast box installed.
Unfortunately the worksheet doesn't let you configure w/ a snowblower or anything between the weight of nothing and the weight of the loader, so I don't know for sure. I assume they are using the arm in addition to the weight to come up w/ a CG...