I've had horrible luck with Sennheisers over the years. I know that's odd, because most people love them. I wish I could put my finger on it, but while I really liked the HD800, I couldn't justify the purchase when I already had some T1s (which I felt were sonically similar, with the HD800s more resolving of detail). I didn't much care for the HD700s. I absolutely HATED the Amperiors. And if not for discovering Grado as my 2nd headphone ever, the HD558 (I think that's what the model was) almost ruined me for all other headphones.
That being said...I LOVE the HD6xx. I have tried several times to get in on the Massdrop buy, but each time I was too late. Finally they just blew it up and allowed thousands to order, and that was my chance. I've had them for the past week and probably have about 30hrs on them. I like to use a rotation, with each headphone sort of getting assigned to certain tasks. The 6xx is definitely a multitasker - fantastic with both music and movies.
I don't know who it is at Massdrop that handles the tuning specs, but he must have very similar ears as myself. I hated the piercing highs of the AKG Q701s and wasn't eager to try another. But the Massdrop 7xx is fantastic. I think the way Massdrop handles these "xx" versions is very smart. I hope all involved are making a killing, because I'd love to continue seeing these sort of collaborations continue into the future.
Hope everyone is well.