Well, it's happened, I'm back to a one-headphone main system. No matter what I buy or try, I can never quite shake the feeling that the ESP950 is as good as it gets for me. After owning an HD650 for a year, I've decided that Sennheiser's former flagship is about as close as a dynamic gets to the E90/ESP950 in terms of sound signature, but the Koss gets rid of the HD650's often frustrating slowness and heaviness of impulse response. The humble ESP950 is like a fusion of all the traits I liked about Grado's PS1000 (speed, air) and the HD650 (warmth, intimacy).
I guess it's time now to start looking into upgrading my E90, since it looks like I'm settled into one headphone for the forseeable future.
ESP950's MSRP is over $1000 (and was even higher than that), it is kind of unfair to compare the HD650 with it.
Try HD700 maybe.