Every few months I take my K612s out and remember just why I love them so much. They were basically my daily driver for several years, across multiple purchases. I'd sell them off, then regret selling them and buy them again. I have them listed for sell, but man, I'm wavering.
They just have a certain character to them that my other headphones don't have, and I can't really nail down what it is. Outside of that something, they still tick a lot of boxes. Great bass extension and control, large soundstage, great imaging, just enough treble detail and sparkle, they're superbly comfortable, and they're very modestly priced.
I was always a little disappointed that they never caught on as the go-to recommendation for mid-fi open headphones. I think they're every bit the equal to the HD600/650, despite being priced well below them.
There was a point where people swore by the K501, saying it was a better headphone (versus the K601, anyway), but I never really felt that way. Having owned the K501s for a time, I was pretty happy to sell them off. They were too bright, too shouty, and had very poor bass extension. They were good with very specific music, but at their best I still didn't think they were as good as the K601.