I spent the last couple of days agonising over whether to buy these or the UE 6000. Fortunately one of my friends has the 550s, so I spent a couple of hours yesterday putting them through their paces with a variety of genres, and I was really impressed. The bass certainly is less pronounced than in the M50s, which is what I was using as a comparison set. I already own a couple of phones with bass humps, and my main criteria with my next pair was a flat and even response; I want a go to pair of phones for faithful and detailed reproduction. I can understand why people might find the 550s tad 'unexciting' if they're being highly critical - there are definitely more fun sounding headphones around. That having been said I personally did not find them boring, and they hit the nail on the head for the reference kind of sound I was looking for.
I'd heard that the UE6000s are a little warmer than the AKGs... That combined with the fact that I really don't want a set of headphones with a plastic battery door in the cup and noise cancelling switches and circuitry (more stuff to break over the years), I decided to order a set of the 550s for £86 + free postage (a really decent price for this kind of sound!) You can actually pick the UE6000s up for £73 on amazon right now, so if anyone else is grappling with the same dilemma I was, that could be a deal breaker.
When they arrive i'm going to borrow my friends pair again and spend a day listening to them side by side (he's been using his every day for 6 months), because I'm interested to see if the treble sounds any different out of the box.
I didn't pick up on the artificial treble sound mentioned by some - that could have been down to my choice of test tracks, my ears, or maybe they calm down every so slightly with use. I'll report back on that if anyone is remotely interested