I also have both and also prefer the DT880/600 to the HD650.
Warning: personal subjective opinion follows.
I tend to think that HD650 is one of the more over rated headphones. It just doesn't have much of anything going for it beyond being merely competent in terms of sound quality. It's probably the heaviest headphone in my collection, and the clamping force is much higher than necessary. The ear cups do fit completely over my ears, but are not as comfortable as the Beyer's. I would say these tie with the Ultrasone 650 Pro for being the least comfortable to wear headphones in my collection.
In terms of sound, the HD650s has neither the detail/transparency of the K70x nor an endearing playful sound signature like the DT880/DT990. So what you get is a pair of adequate headphones with a dark sound signature. They work well if I listen to them, definitely a good pair of headphones, but compare them against other fine headphones on the market I find myself preferring the sound from something else.
If I were to draw a parallel, I would say Sennheiser HD650 is the B&W of the headphone world: great reputation, awesome resale value, but you can do a lot better for same or less.
Jack