Update of my mods:
1. fuzzor - many fine descriptions of this step on Head-Fi - required IMO
2. back screen off, I use 1/2" ground cloth (which is metal not cloth); make sure you cut it accurately, and work it so it fits snugly, not loosely. you have to make sure the 4 slots for the retainer are totally clear of the legs of the screen, or the retainer will not fit. Make sure you don't leave left over pieces of metal or soon to be broken "legs" - I spray painted mine black for looks.
3. before you do the final fit of the screens, add the dynamat AND sorbothane to the back of the can, along the edge where the membrane mounting plate fits the headphone body. I use two lenghts of dynamat and 2 lengths of sorb, interspersed.
4. get a copper cable to replace the rancid SPC stock stuff. Hit the jackpot with an old Black Dragon v1, but I'm sure many would be as/nearly as good.
5. install the Ether angled pads - line them up so that the stators look even through the dust fabric, no that important per se - but means your pads are lined up vs each other.
6. when not in use store them in a gallon plastic bag or the box - since the rear screen dust protector is gone.
7. Not sure that putting felt on the front facing bar does anything but, there might be a placebo benefit - or more.
8. They like watts, not as greedy as the HE-6, but 2 min, 4 better. They like transistors with quick hard bass (Rag 1 suits them).
9. Oops, forgot: I use the 4XX headband instead of degrading stock one - now on my 4xx. The newer HE5SE works nicely too.
10. the above mods have lost me some bass under 60 hz. But the bass is much quicker, not too thick, mids and highs are cleaner. IM is much less. They do sound like they got a dose (30%) of HD-600 added in, but still obviously a 500 without the chubsie mid bass, treble ringing cut way down too.
Rear screen mod and pad change really seem to enhance each other, neither as good alone.