Something else to consider, try dipping your butt slightly closer to the ground during your set-up. The bar is set over your feet perfectly, but as you start the pull, your hips (which have less stress on them) are kicking back (knees straightening out too soon.) My biggest issue with DL is my grip, so I'm just throwing out "cues" that have helped some of the other guys that lift with me.
One of the guys that I lift with has an almost identical DL. He also makes the "S" shape with the bar that you do when he goes to set it back down. He started using a sumo stance, and actually gained about #85 in one session with the different stance. I'm not saying switch stances, but the concept of sumo requires you to stay more upright with your back.
If I was using bumpers, I don't think I'd ever lower a bar again as long as I live.

EDIT:
TONS of DL tips.