The 595's would not exactly be portable. They are also open-back so sound will leak out and in. This means you'll need even more volume to drown out noisy environments, and passers by will hear your music in quiet environments. An amp is not a requirement, but they really wake up with one, you won't be doing them justice. I have the HD555's, which are the same headphones with the exception of a removable foam pad inside the earcups which lowers their sound quality. If you want to try the 595's anyway, go with the 555's, and remove the foam. This will give you the 595's without the price tag. Electronics-expo.com still has a coupon floating around offering them for $75 (expires Oct. 10th). If you want to attempt to use these as portables, you will need to consider changing the connector. They come with a 1/4" (stereo receiver) plug, with an adapter to a 1/8" (mp3 player) plug. This gives you about 3" of connector, almost as long as many mp3 players.
As for the 448's, I haven't heard them, but have read they have good SQ, but are kind of hard to drive, similar to the HD201's. I have the HD203's also. They sound better than my previous HD212 Pro's, are also very bassy (i.e. fun) but they have very little sub-bass. Most of it is punchy around the 80-120hz area, and is not as well controlled as some of their better cans. Mids are adequate, treble is bright but smooth without sibilance or fatigue. They are easy enough to drive without an amp.
If you're a bass head, the most sensitive headphones I've ever owned are the Sony MDR-XB500 (yes I know you wanted Sennheiser). These can be driven to ear-splitting volume from my Sansa Fuze alone, and the bass extends below the range of human hearing with seemingly no roll-off.
Also, if you have Best Buy in your area. Various stores have either the HD203, or the HD448 on demo, from what I've read. Mine, however, had neither, but had the HD201, XB-500 (don't judge the sound from the store), Monster series, Bose, and Koss PortaPro.
