Using the HeadRoom Desktop (maxed out; single-ended), I hear a world of difference among the GS-1000, HD650, and K-701. The primary difference seems to be in the crossfeed, and this becomes obvious only when I do a comparison. The GSK blows away the 650 and 701. It separates and clarifies the entire span, whereas the 650 and 701 seem to glob it. The result is a crystal clear sense of tracking ear-to-ear and mid-head movement. I used an instrumental with complex crossing patterns, Faye Wong's "Uneasy" (Restless, Cinepoly 1996).
Soundstage point-of-view, too, is different. With the 650, I feel as though I'm in the midst of the musicians, up close and compact. With the 701, I'm in the audience, with a wider, higher, deeper soundstage. With the GSK, I'm in among the musicians but on a very spacious stage -- with the added sense that the musicians are actually moving (nearer and farther) on stage to give me the best possible sound (and definition) at every moment throughout the track.
With music that's not so dependent on crossfeed, the differences seem to be far less. The GSK seems to have been tweaked by human hands to deliver the most musical sound. The 701 seems to have been designed to deliver clarity and accuracy. In this scheme, the 650 seems to be a little bit of both.
When the DAC/amp and source are held constant, the cans of choice seem to be dependent on the music and what you want out of it. Thus, head-fiers probably have more than one pair that they shuffle between/among.
Would I sell/trade the 650? No. It's a de facto standard for review and discussion purposes, and for me it's a working compromise between the GSK and the 701. Thus, if I had to choose only one pair, it would be the 650.