It would seem the R4s on the B2601 are skid steer tires and are super for that intended purpose, but not for a tractor, and the book calls for 40 PSI. Mine are loaded, and my fat arse in the seat should be no problem, but they are way to hard and will not grip, the BX has R4s and that little pig will go just about any place in 2WD, but it is a different R4 compound and bar configuration.
Try running them with about 30psi. The secret is to get more of a contact patch.
You can gauge the effective of this by one of Two ways ( this is a tip I use on the track day car).
jack up a tire & put a clean sheet of paper down under the tire while on a very smooth surface and slowly lower the tire down so it doesn't "bounce".
Or you can color a couple of the bars (Kids sidewalk chalk works well) all the way across the tire and drive far enough that it transfers to the payment.
Looking for 2 things....
1). Is the contact patch of the tire all the way across the tire? - If not lower the tire pressure till it does.
2). do you have at least 2 rows of tread in contact with the ground? - Limited what you can do here lowering the tire pressure helps but the circumference of the tire matters more than tire pressure. This is where bigger diameter tires really help.
If you have a backhoe on the back for example perhaps 40psi is what you need but if you do not have anything more than a back blade or something else that is less than 4-500 pounds you might find that you can let quite a bit of air out of the tires and get a better contact patch - thus better traction ( it's cheaper than buying a different set of tire!). Yes R4 rubber is hard and that hurts in the winter some but you might find playing with the tire pressure helps more than you thing it would.
Kubota in the recommendations in the owners manual has to account for the heaviest loads to error on the safe side.
I my case on my LX2610 I run about 35psi in the fronts and 30 in the rear. I could go less in the rear but when I have the stump grinder on it's perfect. Also I am dealing with lots of sticks, branches and an occasional small stump and tire puncture is a bigger concern for me in MY use case.
Hope that helps you.