I've had both (well, the K701, which was rebel-alliance white instead of Darth-Vader black). I liked both headphones very much, found both very comfortable, liked the bass on the 650 better, liked the crispness of the K701 better - and sold them both before moving on to the HF2. If I had to pick one or the other, I'd go with the K701, but just know that it needs a good amp in order to balance out the HF with some solid bass. That's the weakness of any open-air phone. The 650 gets around the problem by using filters to attenuate the HF. I felt like the 650, in all its laid-back glory, missed opportunities to engage me. I didn't like the amount of filtering between my ears and the drivers, at least not as much as the fairly open relationship on the K701. Both of these headphones need an amp, but the K70X needs an amp, and a better amp, more than the 650.
Some people hate the K70X. They find it shrill. They think its soundstage is artificially grandiose. They have other objections that make me yawn. I prefer my Grados, but I respect both of these headphones for what each does right. If I had to pick one, I'd go with the K701 in a heartbeat. That's no diss on the 650. I just like the K701 better.
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