Been a while since I posted here...
I now own all three of the DR-Z line, except the Z5 (which I sold due to it being obsoleted). The review I mentioned a while ago is still in progress; I lost a lot of it to a power outage so I've been slow when it comes to working on it.
However, I will say this much now: the Z7 is aeons ahead of either of the other two; its absolutely ridiculous how different all three of these headphones sound despite having the same driver topology. It is also the brightest of the three so a lot of people wont like it, and unfortunately its the least comfortable (with the Z6 being the clear winner there). But, it sounds so incredible that I don't even care. For me, they're a perfect match. The best I've heard... bar none.