Pros: see review
Cons: see review
I'll try to be brief here, which is nearly impossible for me to do.
I bought this headphone last month and previously had two pairs. Those pairs had really anemic upper mids and had bloated mid-bass. This pair does not. I even used my old reference tracks I tried previously and this pair has a quite balanced sound signature. Not dead neutral or completely uncolored, but I would say it's just smooth sounding. Possibly one of the most musical headphones i've owned.
This headphone for me makes a lot of my crappy recordings that are harsh and fatiguing a bit easier on the ears. I don't ever get bombarded by treble or anything else. No shouty vocals or whatever else. It also doesn't make any very warm recordings worse by making them sound congested or less clear. I've found that they also seem to have better bass extension than my HD-600. Punch me if you want, but the bass on THIS HD-650 seems more controlled than that of my HD-600. It's closer to the bass quantity of my HD-580 (same driver I know).
Despite being very forgiving for bad recordings (only the garbage with the mids/treble cranked to max in the studio) it still need higher bitrate files. You can still tell the difference between high quality recordings and bad ones despite being both in FLAC.
These are very detailed. Nothing is missing, but all that detail is not shoved in my face. As hard as it is to believe, my DJ100 and HD-650 benefited more from my ODAC than the Q701. No idea why. The HD-650 seemed to become more revealing and detailed. The Q701..not as much.
This is a good headphone for when I just want to listen to my music and not be bothered by recording flaws. I can listen to these all day without fatigue.
The soundstage is better than that of the HD-600/598 IMO, but it's not larger. Better imaging etc. I think the ODAC is playing a role here too. Never thought such an improvement was possible unless my ears are playing tricks.
The sound is nice and full sounding, but not excessively warm. I thought I hated warm sounding headphones, but now I guess not.
This pair also doesn't seem to have sucked out upper mids, but at times there seems to be a very slight peak. Maybe a tiny one. I know this goes against what nearly everyone says. This pair also doesn't seem dark. The treble is very good and not lacking. It has slightly less than my DJ100.
Despite making my bad music sound a bit better than it should be, it still is quite accurate to how the recording is..sometimes. You often don't realize how bad the recordings are until you switch to a different headphones such as the KRKs, DJ100 or Q701.
I gave it a 1/5 for value. It's worth $400, but $500 is nuts! My $80 DJ100 with the same setup isn't that much worse and often even better in some areas. Both have some similarities. The DJ100 just has slightly more forward mids and more treble. DJ100 has better low bass. Sounds a little clearer too sometimes. Both are now my top 2 favorite headphones.
This review might be off the general consensus, but I don't care. This one is much more balanced than the others i've heard.
Setup was Headroom Micro Amp (not a portable amp an drives it with ease) from ODAC. I always use FLAC files. I previously used the Asgard with the HD-650, but found the Micro amp way better.
BTW this is as close to my DJ100's sound as I could find. My DJ100 sounds like a weird mix of an HD-650 and maybe a Sextett. Maybe the 650 and another AKG. The DJ100 is definitely a little more forward sounding, but still smooth sounding (with my setup, which I think is very neutral). I only mention this because I always wanted an open headphone that had some similarities to my DJ100.
NOTE: if this was priced at $350 retail it'd definitely get a 5/5 for value.
If you're curious I'd rank my Sennheisers as:
HD-650 > HD-580 | HD-598 > HD-600
Weird, I know..not a ton of difference between the 580 and 600. 580 just sounds better than the 600 to me. Clearer and with slightly more forward upper mids and maybe less bass.
Strange as it sounds, I tend to prefer the AKG type sound, but the HD-650 is just addicting.
EDIT: The only change after use I've noticed is that the lower mids smoothed out a bit. Out of the box they were a bit "shouty". Kind of like the PX100-II (for me) without an amp.
I also think it's a mistake to do many A/B comparisons of these headphones with something that's dramatically different (like a flat studio monitor). With crappy recordings the HD-650 will always sound better and it's easy to be fooled.
Oh yeah.. I also use these for gaming! 40 hours into Dark Souls and even use them for movies. The HD-650 can't match the soundstage of the Q701 though. HD-650 is obviously much warmer and a tiny bit bassier.