I was going to jokingly say that the AD-900s are a suitable upgrade, but I wouldn't carry them around. Now I feel guilty, and am going to try to be helpful.
So, what about the AD700s did you like? Me, I really enjoy the "soundstage" and clarity/seperation of instruments. The comfort isn't bad either. I count myself lucky that I have no fit issues with at AT brand, as so many of my friends do ('they're loose and won't fit,' my friends say.)
I don't have any specific experienece with other headphones, save my Grado SR-225i's next to me. The grados are certainly more portable, and numerous case-ideas have been posted. If you like rock music, I'd recommend them for portable, though they're not very durable, so I would invest in a case at the same time. This is not for everyone, so you may want to get a durable pair of headphones that you can just toss in a bag and not worry. You'll have to rely on someone else's expertise in that field, as I have no experience outside my own possessions. I don't live near shops, nor do I have friends with good equiptment. Though, I'd recommend reading ljokerl's recent review of like 5 expensive portable headphones (that is in the frontpage news-whatever), as it's very well done, and you may find what you are looking for there. In particular, the Sennheiser HD25-1 (or whatever they're called) intrigued me, as he says they are extremely durable, and isolate out noise, two important factors for me, as I walk to class. For this reason, I'm considering them as a possible purchase at a much later date.
Sorry for the Typos. I'm not a fan of head-fi's WYSiWYG editor, 'cause my handy spellchecker won't work right with it, and it's too much trouble to paste it into a word processor.
Perhaps I've helped. Other than maybe a new or different sound, what are you looking for in a portable headphone, specifically.
