I'm actually slightly disappointed. My primary headphone is the Sennheiser HD700, with occasional use of the Hifiman 400i (I also have the Philips Fidelio X2, but almost never use them anymore). I didn't expect the 6XX to supplant the 700, but couldn't resist getting the storied 650 for $200. And you never know, many people prefer the 650 to the 700 (indeed, saying anything good about the 700 on SBAF is, without exaggeration, heresy punishable by digital tar and feathering although there are plenty of people on Head-Fi who love them). I was anxious to hear the 650 at last and have listened to them exclusively over the last few days. They are a very good headphone and I enjoy listening to them; their greatest virtue, as I've often read, is the mid-range: vocals really stand out and are indeed wonderful and listening to them on the 6XX can be addictive.
However, in most other areas, the 700 is clearly superior. The soundstage is significantly wider, instruments are more accurately placed, and there is much more air around them. The 6XX's most surprising flaw is (at least to me) its comparative lack of bass. I'm not a basshead by any means; I never listen to hiphop or EDM (I mainly listen to almost every genre of rock, folk, and pop rock since the late "50s). However, I love to listen to bass guitar and bass sounds great on the 700, even if quantity could be a tad better. Unfortunately, on the 6XX bass is OK at best. On many songs It's often hard to pick out bass lines. (FWIW, I'm listening with an OTL amp that should be very favorable to the 6XX: for music, Tidal HiFi on my laptop>Schiit Eitr>Modi Multibit>Valhalla 2. For blu-rays and streaming video I use the same gear minus the Eitr.) And the comfort factor is significant as well. The 700 is the most comfortable headphone I have ever worn. The 6XX is...not. Out of the box, it is extremely uncomfortable, bordering on painful, because if its excessive clamping force. I'm aware of the fix and have applied it multiple times (and will continue to do so). This has gotten the 6XX from very uncomfortable to OK when first put on , but becoming moderately uncomfortable the longer I wear them.