ive got the hd 558 (got em for bout 140 canadian) and they are a pair of real sweet `phones, im not much of an audiophile but basically the foam mod works like this:
the foam is made of the resistant bouncy foam that you would find a kid using for some craft
sennhieser uses this foam to restrict some of the driver`s movements so when this is removed, the driver can move more,
but that still doesn`t mean you will get the exact same sound as the HD 598 because even though they use the same drivers, they have different grills
the grills affect how the drivers intake air from the outside thus affecting the soundstage
when i compared my sennhieser 558 stoke vs my friend`s 558 modded i found that it increased the bass with a tiny trade off of really specific peices of detail, (it could be just me)
but i find that the stock 558`s are a pair of really balenced headphones and i only plan to mod them when i can get a hold of HD 598 grills
BTW if anyone finds em let me know
About your other options
25-1's and HD 380's may be more for you if you are going to use it for portability but they sound more like the sound is coming from a boxed in source or room while the 558 and the 600 are more surround and natural
the hd 600 has the best sound without a doubt in the group but 600 are strictly for home and hifi use as it has a 300 ohm impadence which is way too much for any portable devices to power without an amp (if you are going to go for headphones of these caliber get the HD650`s for about the same price)
and the sennheiser 558`s probably the best choice, it has amazing sound, and is relativly easy to power with a 50 ohm impadence (regular in ear buds are about 16ohm)
traveling problems with it would include its major sound leakage and its long cable (3M with quarter inch jack, can be fixed if you find a shorter replacement cable, if you find one plz tell me)
