Well, I'll just post my impressions here - this was my PM to walfredo.
I didn't actually spend very much time with the K601s, mostly because I'm getting sick of comparing headphones that sound so similar, and the many variations I wanted to try with the K501 (with the K601 pads, K701 pads, with the K601 and K701 foam donuts - which contribute a lot to the sound). Generally, I can say that they sound much more similar to the K701 than either do to the K501. K601 bass isn't nearly as punchy as the K701, but it has a little more of it. The soundstage is smaller, but more integrated - the K701 soundstage sounds artificially wide, whereas the K601 soundstage is large but complete. This is due to the angled pads the K701 uses, confirmed by pad-swapping on the K501.
Alas, the K601s don't fit on my enormous noggin well, and can't be ghetto-expanded like I did with the K701 (moving the adjustment part up above the fixed part of the headband, a reversible mod). As a result I could never wear them for more than a few minutes at a time without pain or pressure, and I don't think I could ever get a proper seal with the pads unless I held them in place.