I had the foam left over from hard drive packaging I bought way back in 2000, I would think anything similar would work... perhaps not styrafoam peanuts. You could in fact remove the ear pads and stuff them with cotton wool (or similar) from the back, I'm pretty sure there open like that. The idea really is just to thicken them up setting the speaker driver back a little.
Fortunately, I was able to do a successful removal of the felt layer. Sounds like it removed a layer of mud. They've got about 10 hours of use on them, so hopefully they'll sound even better by the end of the week. I've got Jaco Pastorius' self-title on my Zune, and it sounds fantastic with these compared to my UHP336's which lack the bass needed for that album. Although the 700's aren't as portable as I would like, I think they're pretty good for my first set of cans. Now if I can just find some good foam, I'll be all set.