Jazz's final mod for his HD-800s. I have yet to try this on mine to see how it fares.
Now here's the latest implementation of my mod – the definitive one.
In short: A velvet ring on the pole piece surrounding the membrane and three layers of velvet on the bare frame part at the back – with carpet tape underneath/in between to hold them in shape and together.
Maybe the absorber ring doesn't look too beautiful, but it has to be made of velvet to sound like it should. The triple velvet absorber on the back makes the sound even slightly more accurate and lacks the hinted boominess of the Melamine foam. Covering the steel mesh isn't necessary at all anymore – there's all the bass I need without it now. I'm completely satisfied with the sonic result.