I just received the K551's. Being somewhat of a headphone noob (coming from Beat Solo's) I can still distinguish good sound from bad. My frame of reference when it comes to headphones is fairly limited though.
My plan was to replace the Solo's with a much nicer set of cans. Now to my impressions of the K551's:
They are BIG. Bigger than I expected and I agree with HiFiGuy528 that they are not that practical for portable use. You can't let them hang around your neck without feeling you are getting strangled and while the cups does fold, the fold in the wrong direction so you can't make the pads rest flat on your shoulder/chest when wearing them around your neck, it is the back of the headphones which isn't all that smart if you ask me. Maybe there's a reason for it. In any case, it seems odd that AKG fitted these with iPhone controls when they seem more like living room headphones than portables.
Coming from the Solo's I am probably spoiled (in all meanings of the word) when it comes to bass level, but my first reaction was that the sound was a bit thin/flat. The low frequencies were too absent or weak in the sound field. That being said, they certainly sound good and has lots more room and grace than my Solo's, but I couldn't stop thinking that something was missing. I'm not even listening to bass heavy music - I tested them with John Mayer's Born and Raised album. My theory is that neither my MacBook Pro nor my iPhone 5 are powerful enough to drive these headphones. They need a proper amp to play well I guess.
Anyway, I'll return these and look for another set of headphones more suited for my need/taste. I could need some advice for that, but that's probably for another thread.
EDIT: I didn't burn these in at all. I literally took them out of the case and used them for 20 minutes. How much would burn-in affect the sound I wonder? I'm not going to burn them in because I'll return them based on their size alone (although I'd probably be more forgiving if the sound was awesome).
Edited by selsoe - 11/22/12 at 5:43am