First about rattling issue: had this problem with my old K701. All I had to do was disassemble the left driver like so:       and wrap all the wires into something soft but transparent like stretched synthetic cotton ball.
    Received mine this morning. I see no difference in build quality vs K701 but leather headband is more comfortable and black color means no fading issues (my K701 has yellow spots on one side, weird as they spent all their time in a closed cabinet, never under sunlight and pretty much never worn, maybe 10 hours of real listening time).
   Compared with HD-650 and K701 all stock. First impression K7XX is better but this feeling pretty much all disappeared after thorough evaluation and comparisons. Took no measurements yet, just listening.
   Starting from the bottom: K7XX is better then both others not by much and definitely isn't great but better, more extended, fuller and articulate. The other two are both so so just differently.
   For midrange I found what seems to be a pretty good analogy, imagine three girls ;-). One is meaty, proportional but a bit fat, the other is very lean but still fairly proportional (not your 90/60/90 ideal and not some boy-model like 75/45/65, but probably something like 80/55/80). The third one is like a silicone infused barbie doll; big boobs, super thin waist and super tight meaty ass. Imagined ok, the first one is HD-650, the second is K701 and the third is K7XX. Those familiar with the first two sets should understand the analogy while third one is more difficult to explain. At first it seems to be all good but then you realize some parts of the midrange are presented properly but something right next to those ranges would just be missing and make the rest stand out a bit too tall. It's like looking at a pretty face going down to a nice breasts and then what the hell is wrong with her waist, does she even eat? Then tight ass she wouldn't let you touch or hold and then a pair of pretty legs. Dealing with that kind of sound in the long run would probably be unpleasant but that may just be me and others could probably feel different about both girls and cans  .
   Upper end is taken by K7XX again, the other two are so so but in a different way. HD600 is a bit too mellow while K701 is too juicy. K7XX seems to be right in the middle and right where it needs to be in the treble department.
   Overall all three have their issues but I would probably go with either the fat or the lean one, simply because smaller problems of the barbie girl just seem a bit more glaring on the overall pretty picture and therefore to me just feel more disturbing (like an eye sore or a small imperfection that only becomes visible when your glasses are perfectly clean). Take that back, I don't really listen to neither ;-).
   To me Philips Fidelio X2 is a better set, has it's issues mentioned in my review somewhere but sound-wise it sits right in between HD600 and K701, suppose something K7XX wanted to be but...... . Using the same analogy X2 is like a good looking young girl who you can imagine to look just perfect in about 5 years or so if not damaged by silicone, cellulite, crazy diets or fat and/or just too much attention from men. My opinion on both counts could have been different if I was in my 20th  .
   Value proposition; I personally think K7XX price on massdrop was just about right and probably was not such a crazy great deal. Considering X2 is $300 and HD650 is frequently $250 I would see the fair price could to be around $250 with immediate free delivery from say amazon prime. Massdrop is a different story so $200 for the first product with uncertain delivery time was a good decision on their part.
   I can see many folks would enjoy this set quite a bit, just wish more people had more opportunities to better familiarize themselves with unamplified life music so we could all have more of the natural sounding sets.
   Edit: Now since some got so touched by the analogy with female beauty have to add: also wish there was more attention to a healthy foods and lifestyles and less for all kinds of silicone shops or other means of artificial body enhancements that never last. Maybe then we would have less of those who get offended by such comparisons and get overall happier society.