I missed out on the last appearance of Periapt's cables on (Mass)Drop and I won't be picking up any cables in this drop...because I ended up buying direct and these things rock: I ended up buying a 10ft cable in grey with a 6.35mm termination for my lightly-modded HD6xx. The cables feel sturdy and built for a lifetime of use - and despite paying a significant amount for shipping to NZ, I think they're worth every penny.
verifonixI was a bit wary of the foam mod, but it was really easy and - if I remember rightly - is easily reversible. (Personally, I've had the foam out of the HD6xx for a long time and don't see myself refitting it - I think the HD6xx sound much better without it.)
SharkhunterusaMine are Dekoni fenstrated leather pads - also bought from Drop, although it looks like Drop are now doing their own pads now: https://drop.com/buy/drop-earpads-for-hd6xx-and-hd58x I haven't noticed any appreciable difference in the audio, but I find these pads much more comfortable than the stock ones.