I wanted to wait until I was done with the initial on-ears 50-hour before writing this review. Now with that burn-in officially taken care of here are my comments.
1) A word about amps: the Shiit Magni 3 just doesn't drive these cans well -- at all. I have to set it high-gain and turn it to about 3 o'clock before you can actually hear the cans kick. Luckily my Feliks Audio Echo is sitting on my desk and that baby has a great impedance match with the HD6XX and I only have to keep volume dial between 10-11 o'clock.
2) Fit: they are comfortable, but for someone with a huge noggin like yours truly there's definitely clamping pressure. Nothing major, but I don't think I could keep these cans on for more than 2-3 hours at the time (which isn't a problem, really).
3) Weight: they're on the light side. It should not be a factor for neck strain or upper back discomfort
4) Pads: every Sennheiser I've purchased in the last 4-5 years has come with these same set of earpads. They're cushy, yet firm. They're OK, but nothing to rave about.
5) Build: it looks and feel sturdy and well made. The way the hinges of the earcups connect to the headband gives me some trepidation, however.
6) Packaging: meh, at best. The massdrop x Senn box is ok. Inside the case where the headphones come inside of looks a lot more promising than it actually is. The headphones sit inside in a grooved and cushioned setting, but no ceremony or anything special about it. The cable comes already hooked so you're ready to go without any fuss.
7a) Bass: the bass is undoubtedly present when called upon. It can hit very hard with the right song. That having been said this isn't a bass head's pair of cans and they aren't meant to. There's a small sub bass extention, but has to be the right song and you have to absolutely listen for it else you'll miss it. Some take issue with the bass being somewhat muddled and veiled. While I don't agree with those statements, I can see where they're coming from.
7b) Mids: IMO, by far the "the sweet spot" frequency range for these cans. All mid-lows, pure mids, and mid-highs are stellar. Makes vocals, acoustic, rock, blue, jazz and funk come alive.
7c) Highs: they're a bit on the treble-forward side of things, but it's not unpleasant or jarring like BayerDyamics or NuForce EDC3. If anything the highs are wonderfully musical and great detail rendition. Do I wish it was more attenuated? For sure, but it's just nice enough that I can't honestly complain about it.
Overall: Having owned several Audio-Technicas (M40 and M50), couple of other Sennheiser cans and long-term borrowed two pairs of BDs I can say these are my favorite pair so far. Wish it had a touch more bass and a little less highs but again nothing worth docking points against for. The clamping force was substantially more than I was hoping and honestly the main reason I took a star away. It's not obnoxious or disruptive, to be clear, but it does affect long-session endurance. Aside from those, I feel confident you will absolutely enjoy having these on your headphone stable.