My senns got some damage on the grills on both sides. Do you know if its affecting SQ?
Somehow, my AD700(okay, it actually is my dad's) got really banged up on the right side. It looks as if he hit it with a pole, leaving an indentation along the entire grill. Still, they sound just as fantastic as ever to me. I don't even notice a difference between the left and right channels.
However, there are probably sound changes done due to the fact that the grill is designed for acoustic tuning. I would recommend replacing the grills on your Senns if it is at all possible.
At least the drivers still work, eh? Good headphones are almost invincible!
I had dented my HD600 (same one you have? based on what I saw on your profile) back when I used to own it. I didn't notice any difference with the dent, heck, even without the grilles on at all there was barley a difference in sound.
You can take off the grilles and push them outwards, the dent should come right off easily unless it's really bad.