My tilling tractor is a Yanmar YM1410D
with 3 PTO speeds (IDK what rpm they are).
I start off breaking sod in 1st gear with PTO in 1st gear also.Engine (14 hp lol) around 2000-2200 rpm. I make 2 passes like that working about 3 inches of dirt, moist or dry is no different. Two mores passes like that but with the tiller lowered to about 5-6 inches deep. Then I make a 2 passes in 1st gear with PTO in 2nd gear. Then I make 1 more pass in 1st gear with PTO in 3rd.
After all that, the dirt is nicely fluffed and workable by hand for weed removal and planting.
I know it's a Yanmar and not a Kubota, but hopefully it helps. IDK if your PTO has gears or if it's only a 540 rpm PTO. YMMV
ETA: BTW, I'm in Middle TN, with about 2 inches topsoil, 6-12 inches (in areas) of midsoil (IDK name) on top of the hard pack. Also, moist is easier, I mentioned earlier that it makes no difference for me. I was wrong.