Here's my experience with 650s & K701s. I have owned a Headroom Max amp since '99 (and upgraded in '04 with a Headroom Reference Module).
I started with HD600s, found them dull with the Max, so got new 650s in 2004 (with Equinox cable) and liked them better, but still found them
a little lifeless. Headroom declared that the K701s, when introduced, were the new favorites, so I got them and felt they were correct.
With the Max amp, they had bloom and sweet highs and just the right amount of bass.
But then I got a Ray Samuels B52 amp, and the reverse happened. The 650s were much improved, with a magic midrange with lots of substance that I could just stare into. The bass still wasn't as detailed as I would like, but was pleasant and integrated. By contrast, the K701s now sounded
a little bright and squirmy, even after I recabled them with a Silver Dragon cable. So, I got my 650s balanced (APS cable) and they are even better, especially in the bass.
I am guessing the 701s work best with a more reticent amplifier. But with am amp that just takes a solid grip and won't let go
(which I think describes the B52, I guess), the 650s are a great match.
However, my new favorite phones are Grado PS1000s, as I spoke about in the PS1000_Evolving_Review thread, with honorable mention
to the HD800s. Hope this helps...
Edited by rgs9200m - 6/30/10 at 7:52pm