Are you talking about this ring? because my dt 770 pro 32ohms have them. If yours don't, I'm sure you could order some from Beyerdynamic. I do remember reading a post about someone making their own out of wood (or something similar) and actually improving the sound of their cans.
Anyone open the DT770 LE's up? I've been wanting to experiment with them to get more mid range and less sub bass boom. I saw a mod thread which supposedly did this, but part of it was replacing felt around a black ring that is behind the driver. I opened my LE's up and there's no black plastic ring behind the driver. Every other mod thread about the dt770 I've seen has the black ring. I'm guessing maybe the 32 ohm version doesn't have the black rings? So, I guess no felt mod for me. I did stuff some polyfill into the cups since I had them open to experiment. Seems to have tamed boominess. I need more listening time though. Anyone? Thoughts or ideas?