I just sold my K601, which many critics now view as more balanced than the 595 or 600s tone-wise (see headroom review as an example), but on par with them (Some folks view balanced as kind of boring, so that's not a +/- statement, it is what it is).
I bought the 701s, which are comparable with the 650s. All I can say is, Holy Sh1t. Seriously, if the 650s are to the 600s what the 701s are to the 601, then they are worth the extra dough.
I've heard a lot of cans, and I am amazed at what you get in the 701s...to my ears, they are better than the GRADO 325s, and cost much less. They are amazing. I wouldn't have believed it (even though others I know have raved) if I hadn't heard it myself. To me, they are not the best mixing cans I've ever heard...the closest thing Soundstage-wise, and bass, mid-detail-wise to monitors I've heard.
If I did a lot of loud rock, I might go for Senn 600s instead, or pick up a pair for that...but for what I do, these are pretty incomparable. Actually, as I write this, I would like to own a pair to round out my collection...just because they are a nice set of cans, and especially since so many others listen on them...many of my friends who provide feedback on mixes, for example.
It's almost a little weird...I mean, I did a recording last night through my API A2D...stereo guitar...I put the 701s on, and between picks, I could hear my friggin' guitar strap creaking on the button.