Just got mine this morning. Been listening music for a good half an hour, but these were instantly groovy! You're right in saying that they're definitely AKG, but to me seem a definite upgrade to the K272HD, which I was using on my way to the post office. Awesomely tuneful bass presentation, smooth sailing with the music across the register. I have no trouble distinguishing harmonics. Modal jazz is a breeze, Duke playing those low volume synth things on Zappa albums... Fantastic, everything's there.
Ah, might be new toy syndrome, but I've been instantly disappointed with many other purchases: in other words, I know what I like and don't usually let anything fool me outside of context, which is the music itself.
Can't wait for these to burn in! ... In other words, gain more experience with different music with them and learn to appreciate them for their qualities even more.
Ooh, very musical! ... Just have to add, these are probably not for Rhythmdevils. Bass is very linear and notes are distinguishable, haven't heard better bass-reproduction in any closed headphone. I'll try to compare these to the 8400's at some point, since they have some really good points. These are much smoother though and the bass production of the 8400 doesn't come even close I'm safe to say.
I've never heard the K272HD, but I did own a set of K271MKII which many sources indicated were the same (with different accessories e.g. cables included.)
The K550's are immensely better. Especially the extension at both ends, but in other ways as well.
As for the KNS8400's, I sold mine after buying the K550's.
I do think the 8400's are good value at their price point.
Also like your comments about the bass on the K550's - must agree it's tuned very well.
Very "there", but not overwhelming. Controlled and articulate.