I happen to own the K701 and the HD595. (And have briefly tested HD650s and HD600s). First off, both are excellent. And very different:
K701: Natural sound. For classical music and voice, they sound the most similar to what you would hear in front of the orchestra. This is the utmost quality to me. In exchange for this, and maybe because of this, they are not extremely brilliant at percussion, nor at the highs, nor at nothing. But a piano sounds more exactly as a piano than anything, and the voices, and also drums sound probably more like real drums (but some people might think that this is not enough 'drummy'). They have to some extent the quality of 'soundstage', which nobody can define, but everybody recognizes when it is there. IMHO, sound appears to spread somewhere around the center of your head, you dont know exactly where. Apparently, this induces you to imagine you are before the artist... This is I think the second most important quality.
An added advantage, they are not at all tiring. You can listen at them for hours, and your ears never feel hurt nor 'overloaded'.
HD595: Slightly more 'brilliant'. They often happen to be more appealing to non-experts. Might result in some ear-tiring after some time (though this is also a matter of personal taste - or resistance !). Bass sounds more bass and brilliant, and so does voice, and piano, and violin, an all. To me, the 595 is a moderate laid back version of the 650 or 600. As for soundstage, well, there is clearly less., but it is difficult to tell until you compare...
It all depends on who, for what, for how long, and so many variant parameters.. To get some objectivity at evaluating a music reproducing device, it must be judged as for its ability to reproduce as exactly as possible the true original sound. That's why it is good to test by listening to properly recorded voice or a classical instrument. Then you can compare, since you know what is the real original sound... To this end I prefer the piano, but any other will do.
And now let me make a guess: you'll probably end up owning both ! The question is - which one first ? ;-P Try to test them before purchase. These or any other you intend to buy.
Cheers, hope to be some help-.
