Jerry,
The official lift capacity is about 400#, but my B7100 that is about the same as your 6100 can lift my 525# finish mower. The front end gets a little light, but I only pick it up when I cross the gravel drive.
I don't know if the published lift capacity is for safety purposes or is the actual tested lift capacity. I try not to exceed the published capacity unless I am moving across level ground at low speed.
I have front end weights that would keep the front end from getting light, but I have adopted the same philosophy for the Kubota that I have for my 50 years old Allis-Chalmers Model B: If the weight on back of the tractor makes the front end get light without weights, then it is probably approaching too much weight to be lifted by these little tractors anyway.