While I do not have a Kubota, I have turfs on my JD 4100. I do not even know what the max psi is on my tires, my manual says 10psi in the rear and that is what I use. At max speed in low range (HST) my tractor rides a little rough, but not bad enough that I am happy to get off after 3-4 hours of mowing. I would guess that the max is too high and would certainly give a rougher ride. If, or when, you lower the tire pressure don't forget that you will also lower you mower hight for any given setting.