Sennheiser fanboy?! Never been called that. Absolutely not here, at least. Couldn't stand the HD 600 for a while. Everyone knows how I feel about the HD 280. Currently have three in the collection, but I've owned far more AKG models at this point. I've just been able to consolidate with the K702 AE which, in my opinion, is the most attractive version. I'd actually probably call myself an AT fanboy because I love their signatures in general and still completely ignore the fact that their headbands designs are the worst. (shh, don't mention this in any of the AT threads I'm participating in)
Onto the K712, I purchased one several weeks ago and owned it during the duration I was lucky enough to demo an HD 700 for about a week. Long story short, I returned the K712 and went out to purchase my own HD 700. I probably would have kept it had I found significant differences in the signature from my K702 AE, but I didn't, so off it went. Now, I don't think the setup I was using to test was garbage by any means, but maybe there is something I missed. It was only a week, after all, but I don't think so. I'm very very familiar with that line's signature and it didn't take long for me to make a decision. Keep in mind that's not saying it sucks. Far from it. Anyone who runs into me in the threads knows I'll call out a K7xx as something to watch and own because I've gotten a fair amount of joy from them. In this case, with my current cans and what I was looking for, I just couldn't defend keeping them.
So the HD 700, I'll give you a better idea why I bought my own pair. First, know that I enjoy signatures on the slightly brighter side and I would call myself a soundstage whore. (remember, I'm an AT signature fan) Well, I find the HD 700 is far better at imaging and separation, the soundstage is more expansive, and the overall response is more flat. Yeah, I enjoy the extension of the HD 700 on both ends over the K712. The decay is also extremely quick which, to my ears, makes it sounds more accurate and can be a lot more fun to listen to with certain types of music. The comfort on a K712 is pretty good considering the evolution of it over time, but the HD 700 is just invisible on my head. Value, just like everything else, it's dependent on your preferences. I would say the K712 is fairly priced ≈$400 and the HD 700 is fairly priced ≈$650. What that does illustrate is that I enjoy both, but, in my opinion, the HD 700 is a significant step above the K712.
Now, my mind can be changed if I'm given a good argument. That being the case, challenge accepted! I'm in Pasadena, work in Long Beach, and you're in Huntington Beach. Look for that PM soon!