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.