My best success at taking out small trees is to run up against them a couple inches or so above the soil and upcurl once the front of the FEL bites into the bark a bit. I often need to make 2 or 3 tries at it, because it has to be just so to get enough bite and upcurl at the right point, but I often get them out of the ground, roots and all. Or I mangle them enough so I can back the brush hog over them. Tighten up your top link on your brush hog if you really want to do them in. Just drop it slowly over it. Your brush hog should take out up to 3" or so.
I haven't had much luck digging into the dirt very deeply with the FEL, even with the tooth bar in place. It takes a whole bunch of hp and probably more weight to get that big thing down and back up, with it stuck in the dirt.
Ralph