The k550 is a reference headphone to a large extent in my view.
1) Firstly it's properly closed. Properly isolating. Properly non leaking. And in this department I'd call it of reference quality
Not some audio technica 400g monster that leaks a fair amount and isolates little. And is stupidly uncomfortable
Not a denon that is virtually open
Not a shure 940/ ultrasone pro 900, where deliberately the manufactures have comprimised at the isolation front in favour of the sound quality.
The akg k550 has gone through none of these comprimises and hence they call it 'portable.' It's no way portable in the direct sense, but for it to be used on a plane OR A LIBRARY say from an ipod- that's definitely acheivable. Repeat, they are not meant to be worn when jogging. This isn't what the working definition of 'portable' reads as here. Anyway, it isn't nothing new by the way- beyerdynamic have played it the exact same way with their t5p and t70p headphones.
2) Secondly, to find the fit was a bit picky for me at first. I insist that when the bottom cups press against the underside of my ear they sound much better than when I feel them pushing exclusively above my ear. More bass, more impact, greater dynamics and the notes become more thicker in general. Their ergonomics are wonderful though. Extremely comfortable for all head sizes and ear shapes.
3) Thirdly, they are in no way overpriced in this market. I bought them new from amazon uk just when they were released (no pre-order) and it cost me £229. SQ wise, they smash the living daylights out of the £160 dt770/600. Easily beat the shure 940's and the immensly dissapointing £800 beyer t5p. They beat the £130 akg k271 in musicality and (from memory) the a900. I couldn't get a fit with the ath w1000 so i returned these fast (awful ergonomics and isolation, but okayish on the leakage front). There are other true closed headphones it competes against i know, but don't care to find out about these. Only the £1300 ultrasone edition 8 beats these- after the pads have been throughly worn in though.
4) I can't understand why akg seems to have become close to a bose/beats. Admittedly I was peeved off about their delay in release, but I've got much more anger towards beyrdynamic. £800 for tosh is a bit too much I think. The bass holes in the dt770/600 make it compete with beats pro headphones. That'd probably be a good contest.
5) I can understand why there has been an underwhelming interest of these on headfi. Not many of you guys seem to need a truley closed full size headphone. If you don't need the isolation and non leakage, then stay away from all those that happen to have these characteristics- as to achieve this there has been a comprimise in SQ somewhere.
I'm impressed with the akg k550. I have no reason to be biased towards them, only reason to be biased against them- for being awful with the release date of this headphone.