I use HD650's at work. Love, love, love, love, love them. Sublime sound quality and extremely comfortable for hours on end. And I wear glasses, so I'm very sensitive to how headphones fit on/around my ears. I wouldn't trade the 650's for anyting. Well, except maybe HD800's.
I've never heard or owned 555's, but I've owned a few other Sennheiser cans over the years. I've always been pleased with their acoustic performance and build quality. Whether they were $25 PX100's, or up to the $500 HD650's, they've always surpassed my expectations for the price.
IMHO, to one degree or another, Sennheiser's do impart a little bit of midrange warmth to the sound. They're slightly more "B&W" like than the flat-as-Kansas frequency response that we're used to in our Axiom's. Some might dislike that. I like it in headphones, as I feel it helps establish a more "intimate" feeling in the music given the limited amount of soundstage & depth that headphones can create.
If you want an Axiom-like sound in a headphone, look to AKG. The AKG's I've heard (particularly, the 701's) sounded like having M80's strapped to your head. Extremely detailed, extremely clean, extremely flat frequency response. Wicked-good headphones.
