I've put literally at least a thousand hours on (each) HD600 and DT880. My closest friend owns HD650 and we've traded for weeks at a time. I prefer my modded K702. It's what I use the bulk of the time. The other cans mostly just sit around, so I figure why not sell them?
Never cared for planars, including speakers. Just not my preference
I've owned state of the art systems for decades; downsized a few years ago. Flagships do nothing for me; I only care about the sound. Who knows, I may llike how the K812 sound, but that's a lot of money to take a chance with, especially when I'm really happy with the K702. Just don't feel like I'm really missing anything. Clearly, I really like the K702 and if all else fails, I'll likely go for K712. I've been trying to think of a high-ish-Z can that I'd like and have come up with absolutely nothing other than HD700. Not terribly confident about those, either, to be perfectly honest. Where does that leave me? X2 sound like a possibility, but low-z. Maybe a T90? I auditioned them for a while and didn't walk away terribly impressed, plus if that were the route to take, T1 can be had for just a bit more.
Heh, I suppose I could change thing a bit and opt for K701, but the K702 is a better headphone - matched drivers. I'm completely out of ideas. Almost feels like I'm trying to talk myself out of the K712. That would leave me with DT990pro as the high-z 'phone, which I like a lot, and 900x for the summer. I'd sure like something different; the K712 seems like such a predictable choice. I dunno, maybe I like the modded K702 so much, because they sound a lot like my speakers and I'm on my third pair of those. lol