AKG-Q701 and Beyer DT-990 250ohm.
Going from one to the other requires about a half-hour adjustment period where I think "these have no bass, what the heck?" for the Q701, or "man, these have way too much bass" for the DT-990 250ohm; but I've come to really love their sound signatures, across genres. They both have great sound stage; the Q701 is wider, but the DT-990 has very impressive spacial imaging as well. My HD-650s are still my favorite headphones but they just straight up lose to the Q701 and DT-990 when it comes to soundstage and require some degree of crossfeed in order to make up for their tendency to be "left, right, or center" as far as placement goes. It's not THAT bad, they still have world-class sound quality, but a direct comparison places the two currently under comparison much better for sound stage, each in their own way.
The Q701 have a snappy feel to them, the DT-990 have a more measured presentation. Q701 remind me somewhat of Grados and Sonys except I can listen to the Q701 all day long while I've never heard a Grado or Sony headphone I could stand for more than an hour without getting severely fatigued from the prominent treble. The DT-990 are a lot more like the HD-650s; they are by no means "slow," that would be a pretty silly thing to say considering that even the most expensive loudspeakers can't compete with the slowest extension/reflex drivers at this size, but they don't have the "snap" of the AKGs.
The Q701 are overall a brighter headphone with a more analytic sound signature, while the DT-990 are more even but without sacrificing treble clarity. I do not find the DT-990 to be booming headphones, they're pretty much true to input. The Q701 are true to input as well but can starve the impact of low notes in a way that is initially offputting.
Finally, I am not a burn-in believer for drivers (I feel they arrive at their nominal state within a few hours of normal use, and past that it's just expected wear on the driver that occurs over a period of years, not hundred hour marks), but I definitely believe that ear pads (especially gigantic ones using foam of variable density...) have to wear in to accomplish a truer acoustic seal to your head. The DT-990 have sounded exactly the same ever since I got them, because the clamping force and the unbelievably comfortable and plush earpads have been comfortable since day one with no significant changes. By way of comparison, the Q701 earpads were uncomfortable for the first week or so, and became more comfortable and form-fitting over the course of a couple weeks after that until they have now more or less "broken in" to my head shape, providing superior coupling with less leakage and (and far more comfort - it was painful to wear them after a couple hours initially, now I can wear them for 5-8 hours no problem).
Other differences are largely aesthetic, but that matters too. The Beyers have a much more conservative appearance. They're headphones, they're well built, they're traditional in construction and offer what you need. The Q701 look like some crap out of the original Star Trek. I don't care for the bumps on the headband though they are no longer painful to wear; still, whoever came up with that as a design in the original K701 did a bad job in my opinion. I don't like the thin elastic tension "springs," they seem destined to fail eventually. Luckily I've got a pretty big head so when they do it'll just be business as usual as far as my head and ears are concerned. The modern dual flexible bar thingy, I'm agnostic toward. I don't feel it offers substantial improvements over a well-made normal headband, but I also don't think it's a big deal. It does make storing them a bit more of a challenge, though, I feel uncomfortable leaning them on anything whereas the Beyers' spring steel headband seems like it should take whatever I throw at it gracefully. Definitely more rugged, with less gimmicky stuff and more tried and true construction.
That's my favorite dichotomy at the moment and as a result, even though I prefer the HD-650, and very much enjoy the ATH-A900 and DT-770, nonetheless I've been swapping out the AKG Q701 and the Beyer DT-990 as my main cans lately. 
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