Yeah these little tractors really require weight in the back when a loader is on, even with an empty bucket! My Kubota doesn't have filled tires but I always had a
ballast box on if there wasn't a rear implement. But the ballast box was on the 3520 so I tried putting the SSQA Kubota bucket on the Deere (which I have converted to SSQA) but it won't fit on, they're not quite compatible. So I thought I'll just use the L2501 "it's only wood if I'm careful surely I can carry that w/o moving the ballast box". But the back end came up so that was it.
Just to give you an idea how much ballast we really need on these tractors I'll share what I read in my Deere loader manual last night. This is for a 3520 which is pretty close in size and weight to an L01 Kubota, around 2,900 pounds bare tractor plus loader.
Required minimum ballast for 3X20 tractors: Fluid filled tires, and 1,100 pound of 3-point ballast, and (3) 55# wheel weights per wheel.