I've been burning in my phones for quite a while now.. no changes that I can detect. I have to say that I find the HD6 "Mix" to be a joke for what it's marketed for, just like someone said before. Even if it's a bit more balanced than HD7, it's still a muddy muffled mess compared to even my 4x cheaper trusty Aurvana Lives where you can actually hear the highs etc and everything is much more neutral/open. Maybe the intended "studio" work is actually more for noisy concert stuff where HD25 etc are the norm..
no if you think they are muddy then they are not for the studio