You can, but this will invalidate the 2 year warranty. Not a good idea really because they have weak plastic construction above the ear cups that can crack. Plenty of 555's that have had this happen, if you do some searching. Partly why I think they are very over priced at £90.
More to it than just a piece of foam though it seems. Quite a few people have tried the mod, and said they still don't sound as good as the 595's. It does improve them though, according to many people.
Probably down to people's own preference really, as not everyone has found removing the foam improves the sound. Some feel it takes something away from the sound.