I just got mine yesterday. A few quick impressions:
The fit for me is perfect. I experienced the "suction cup" effect immediately. I'm not small (6' 5" or 195 cm), and by no means have a Beetlejuice head ;-) I had to set each side on '3' (i.e., 3rd notch; I love that the band is notched) to get the right fit. But I can see where an improper fit would cause problems.
My only other decent cans are old Grado SR225s. I like their sound, but they just aren't that comfortable. I am going to try the Senn. HD414 quarter mod with them, if Amazon can manage not to ship the actual Grado L Cush pads again. My main reason for getting the K550s is for my iPod Classic ("7th" gen 160 GB). I don't have a HP amp... yet, but it's on my wish list. Until then, I wanted cans which could be driven by the iPod. The Grados distort a little, esp. on mid-y, treble-y indie rock. I tested a few tracks with the Grados (brand new L Cush pads), and then the same with the K550s. What a difference! The K550s give so much more presence (better sound stage, I guess), and the details jumped out at me. And zero background noise (thanks, Good Fit). I did notice a little harshness in that suspect band (4-8 kHz, or so I've read). Some jagged guitars. But other tracks (anything from the remaster of Rush's Moving Pictures) were deliciously clean and detailed. The low end was plenty for me, though I've yet to test with anything seriously bassy. OK, I did queue up some Kelly Clarkson disc (not mine, I swear ;-) which has some decent bass, and thought it more than adequate.
Which leads me to the burn in process. I'm hoping that they will smooth out a little over time, and given reviews here and in various magazines, I assume they will. In a general sense, what's the best way to burn these in? I've had them plugged into an M-Audio Revo 5.1's HP jack, running nearly continuously for 26.5 hours now. The volume is not quite maxed, but the Revo's Output Mixer never shows clipping. At first I ran some seriously noisy stuff (a couple Flaming Lips albums), mostly because they seem to hit the troublesome frequencies a lot, and will play LOUD, with that Kelly Clarkson in between them (and set to repeat in Winamp). After running that overnight, I switched to a far more eclectic mix, also on a loop.
Is it OK to run them continuously (sure would save time), or should I let them rest? Do cans need rest?
Also, any recommendations for a cheap (below $100 US) amp that will work well with a Rockbox'd iPod Classic? Given the nature of most of what I listen to, an amp that might warm things up a bit would be ideal. But I'm truly clueless when it comes to HP amps. I'll be doing a lot of reading now, as I really want to get the most out of this setup. I'm more inclined to a "desktop" type (runs off an AC adapter, rather than battery; I think I've got that right).
TIA for any advice and feedback.
In short, I will NOT be returning my lovely AKG K550s :-)