non sibilant highs, crunchy mids, loads of air, non constricted soundstage (unlike denons here which make the soundstage feel too defined with borders, the k550 allow female vocals to literally extend into the air surrounding your headphones). Lovely defined fast midrange with plenty of speed. Very good attack and decay.
As i've said before the main problem with the akg is if tonality as it sometimes don't sound natural. That's perfect for trance music though. Mind you they rock at classical, jazz and ambient music too. Easily better than my denon d7k in these genres. The m stage is very impressive (with the hrt music streamer) with the k550.