I'm not talking about noise floor. I'm talking about that black space, dark void, instruments appearing out of thin air. You know...When the first note hits it is a surprise. I don't know how to explain it properly. Lack of that is not a complain towards k550 actually, even if akg's were sold for full price I don't think I have heard a headphone that can achieve that sensation for 300usd.
I don't know what is the cause for that. Maybe it has something to due with the faster decay of notes (that would rule out dx1000 which even has reverbs) or something... Overall lack of transparency. So...just to clarify, it is not a hum, or hissing or anything like that. Just the lack of overall credibility that music is presented from somewhere out there.
EDIT: And like I said I did not espect such from K550 but that people seem to compliment it for such performances so I just offer different opinion. K550 is just a fine capable headphone, but IMO just a bit overpriced compared to what it offers.