Are you sure you want to cut them off and leave the stumps? The stumps are a nuisance, IMO. I'm removing mine by either pushing them over and ripping them out of the ground with the grapple, or by wrapping a chain around the trunk and pulling them out of ground. For the larger ones I can't push over with my DK50SE, I dig up the lateral roots with the BX23 backhoe and then push them over or jerk them out with a chain.
Question about pushing them over: Grapple high on the tree for leverage? Grapple curled back or rolled forward?