I can sugest a very easy mod. I don't think it is as effective as previous.
Last week I noticed that my HD800 become more brighter and their soundstage become a little bit blurred and flat.
I thought it was because their earpads become a little bit crushed up.
My purpose was to hold away diaphragms from ears and make them see into my ears at a larger angle (so that they sound more like speakers).
So, I find earpads of my old akg k26p. And cut out pieces of foam rubber and cut them (I just made them somewhat shorter) so that they can find enough room for themselves under HD800's earpads. You can also use any two pieces of foam rubber the same size.
Then I insert them under back side (which are behind ears) of HD800's earpands. Now it looks like that:
After this the soundstage becomes slightly deeper and it moved a little bit further forward.
Bass becomes more legible in my opinion. It also seems that headphones now sound less brighter and more comfortable.
I didn't notice any negative effects in sound. Maybe the transparence is now a little bit worse, but I think it is because sound becomes less bright.