i just discovered this "closed headphone" vs "open headphone" thing. i am reading that open headphones have better "soundstage" than closed ones (leaking sound is not a problem), but the bass is potentially less? did you notice less powerful bass in your HD-555 compared to HD-280? i am not expecting earth shattering bass, but i wouldn't want it less than my existing headphones.
i had my audio technica headphones since 2006; since I am very careful at handling my headpones and never move them away from my table. so i am thinking of splurging on the Sennheiser HD598 since it looks like it might last for quite some time, what do you think?