Standard Series PTO Stump Grinder

Quick shipping and great packaging. My first stump grinder. On set up learned I had to cut the driveshaft and was having a bit of confusion about protocal when wife said ck you tube and found same machine through MaxMech their procedure was simple enough for a lawyer to understand, so after other checks i was ready for first stump and chose a small dead one no problem at all , so on to a larger fresher one I have a 37.5 hp kubota with regular transmission, even in lowest of low gears the tractor seemed to move too quickly across the fresher hardwood stumps... so i am figuring how to go slower or take less of a bite of stump. I noticed the competitor's machine manual stated you needed a hydrostatic transmission to use the grinder so I am not sure about this?? If I try to use the clutch to slow movement it seems I lose driving force to the pto. I will revisit the issue and add to reveiw or submit additional info to let you know what I come up with. Tim M