Cheap answer: http://www.bhphotovideo.com/c/product/886679-REG/Sennheiser_549375_HD_580_Headband.html with 2 strips of double sided permanent tape from any local store. Will have to replace headband pad eventually depending on use. Cheap foam deflates
More involved answer: Remove threading from leather pad, remove bottom replace current foam with much thicker denser open cell memory foam or some equivalent, restitch and never have to replace again. May or may not void warranty. As it does not physically do anything with the drivers, I'd say no. But asking head-direct may be best.
Doing the latter suggestion here soon, just haven't decide if I'm going to replace the leather entirely with some other color. Will post results when I do finish the project however!
As for earpads. After trying a few different pads. I settled on DT 770 pads. 1mm size difference from stock pads so assembly takes minimal stretching. Also I very much like the sound without the dust cover between my ears. Just sounds better to me. Less veiled. Slightly similar to removing the outter dust cover as some has tried. Just not quite as strong a difference. My 2 cents. Ears always very!
Thanks for the suggestions NoEars. I have actually tried the foam strip from the HD 600 already and while initially they weren't bad. After some extended use, the foam compresses (like you say) and actually becomes quite annoying as you can feel the individual pads touching your head. I have since taken the foam strip off and am back to just the plain headband. I'd be interested to see how your modifications go as it sounds pretty interesting.