If the HD 6 Mix can play in the same ballpark as the SRH1540, MDR-7520, or Focal Spirit Pro, then yes, they will be a no-brainer for me as I am a bit of a Senn fan. I have only read about the 7520 and Spirit Pro, but I have heard the 1540 and it is excellent. I wouldn't call the 1540 bass heavy, but I would call it warm and emphasized bass. I need something for home studio work, like an HD 6 Mix, but I still want something with extra kick in the bass, just not too much, when I need a closed headphone that is just for fun around the house (I have HD-700, which I would also describe as a fun rather than neutral signature, but in a different way..and I am tired of my family telling me to turn it down when I want to really cut loose, so hence my interest in this thread). I like most of the Denon models I have heard, and I really liked the M-100.. so again, if either the HD 7 or HD 8 are along those lines without getting super bass heavy then they will fill a currently empty role in my headphone stable. I like the metal components and style of the HD 8 slightly better than HD 7, but if the HD 7 was even slightly less bass heavy than the HD 8 I would go for the 7. Looking forward to more comments, reviews, etc..
The 6 is supposed to be more neutral of the 3 and if the bass on the 7 is any indication, the 6 should have ample bass as well.
The build quality of the 7 is amazing - when I opened the case I thought they sent an 8 - very robust design even tho it's 30g lighter or something like that. I think durability will not be a concern. They are tanks.