I dont think you would damage the FEL by hitting a stump. I used to run my ol Yanmar with the fel down low for just that reason when I first bought my place. It was overgrown with brush and weeds and I did hit a cedar stump. No damage to the FEL. Once I was clipping along in 3 H range (about 8 MPH) and hit a sink hole dip that was about 3 feet deep but grass coving it was level so it wasnt that obvious. Must have been an old tree root ball that was removed or something. ANY way, the FEL dipped and plunged into the ground. Instant stop. I about went over the steering wheel, but no damage to the Tractor or FEL. They are pretty tough. I used mine to push over trees, ride the tractor up till the front wheels were 3 feet off the ground and never hurt it. NOW you could spring them up if you hang on corner of the bucket under a root or something and get really agressive with lift it up and pushing in 4 WD. My Yanmar V3 is pretty darn tough and the LS seems to be even stronger built. The only thing with the V3 is trying to keep the bolts in it where it bolts to the tractor frame. They just keep getting loose and falling out no matter what I do to tighten them. I have a couple that I have to replace in it now, before I can use it again.
I just keep my bucket level so that if it hits something it hits with the blade edge straight on. That is the strongest position. If you had the bucket curled up so the bottom if facing forward, you may bend it if you hit a stump hard, but if you keep the bucket level, I think you would be ok.