If I could tote an elephant-sized headphone on my head for the best portable sound quality, I would do it.
I do not wear the ATH-A900s portably, and I love them.
Here is why:
1)V-wing supports do not clamp down upon your head, allowing the pleather pads and cups to jump up and down on your ears as you walk. This is quite an uncomfortable sensation and impractical for a pleasant listening experience.
2)They're HEAVY. REALLY HEAVY. Heavier than the plastic sennheisers, heavier than the MS-2 (I believe), heavier than anything I have ever worn.
3)Even with the best cord wrap out there, it's difficult to manage a ten-foot long cord.
You could wear the HD280s portably if you wanted to; they're fairly light and small.
However, I don't see why you would; the sound quality provided by Koss Porta Pros or Sennheiser PX-100s is just as good, if not better!
I don't like the HD25s enough to purchase them over the Porta Pros or PX-100s, either. I don't see how the cost is justified...you are paying a lot of money for a minimal improvement in sound quality in a closed headphone.