Selective frequency and ringing damping:
I've spent time testing this mod on Slants and HD600. Rip out eighths of a piece of toilet paper. It's easiest if you roll up the piece and then rip the cylinder looking thing into eighths. Ball up one of the pieces and put one in each ear pad in the same place. Play your favorite test song and move the pieces to different places. When the music starts to become more alive, and you feel the sound is improved, you can firmly push the pieces into the recesses of the earpad. You may double check with a sine sweep. Note frequency flattening, filling in of dips, damping of peaks, better articulation overall or of certain instruments (by timbre or frequency range), changing timbre of the sine sweep, and perhaps better channel matching. Repeat as many times as you want. In general, more toilet paper will make a warmer sound. This happens most severely when compressing and cramming toilet paper indiscriminately into the earpad space. Whereas a lot of uncompressed toilet paper causes a veiled sound and a flat sheet flattens at first and then causes a broad hill somewhere in the treble, upper mids.
An easy test for whether the damping place is correct is if you end up still listening to the headphones instead of ripping them off in disgust or pain.
I was able to fill in the 10k-15k dip and ~20k spike in the HD600 a bit with this. I presume this will also work with other headphones, including the HE1000 and its current splashy treble.
Note that I have the front foam behind the earpads and toilet paper/black cotton mesh damping in the grills.