Got mine today! Serial 1451. I think they attempted delivery a couple of times already though when I was on holiday and the office was closed, starting from before new years. No customs charge via USPS and Canada Post which was nice.
First impressions are that I'm enjoying the spacious airy sound as expected, and also that the bass isn't at the levels I enjoy for most of the electronic music I listen to (also expected). However they excel with music with instruments and lots of midrange and treble detail and the bass still goes low and is present enough especially when they are turned up volume-wise or when the surrounding room is quiet.
Comparing to my Ultrasone Signature DJs (sure, an unfair comparison in terms of price and closed vs open), they are super light and comfier. Much flatter sound signature of course. Less exciting. Not as much treble sparkle or detail up top, and the Sigs are just a perfect seamless enjoyable smooth headphone that are blissful to listen to. Still, the K7XXs are going to be enjoyable for sure, and I mainly got them for gaming at home on the PS4 and a bit of music listening. Definitely a bargain. If I ever get back into music production again a little, I'm sure these will be excellent for tracking, mixing and basic headphone mastering.
These are my first AKG headphones and first decent open dynamic headphones (last ones I had were crappy but comfy Steelseries Siberia's). One thing I'm surprised at is that they are actually usable for office use at moderate volumes - they don't leak too much. I guess the last true open headphone I owned were HE400s which leaked SO much (basically just speakers on your head), and even the semi-open TH600s leaked a lot.
The downside is that a really good headphone like the Sig DJs or TH600s sound amazing even at really low volume. To me the K7XX are a little muddy until cranked up a bit (probably the lower level of detail retrieval). But its probably also the fact that they're open and depends on room noise levels.
Only other comment is the build quality is obviously pretty cheap and flimsy. When I move my jaw or head around it creates some creaking/rubbing noises with the plastic/pads moving against eachother, which is a little distracting..
Quick phone snap: