Pros: good frequency response
Cons: muddy bass, soundstage not very wide, slight veil
I recently used a borrowed HD650 in a balanced system for about a month.
After I gave it back and returned to my balanced Grado HF2's, I was shocked how much better the Grado's were.
I must thank Sennheiser for making me appreciate my current headphone. That's about the only good thing I have to say about the HD650. :) I'm kidding, it's good but if you value an open and fast sound this is not the headphone to get.
I would venture to say someone with high frequency hearing loss would be the one's to gravitate toward Grados overly bright signature!
Here's the HF2 graph vs. the HD650 graph:
http://i764.photobucket.com/albums/xx290/sinf/casti/HF2vsHD650graphs.jpg
your grados hf2 have a nasty spike at 10k, for me it's trebly.
On the other hand, the HD650 is average whatever you plug it into, that's why people put silver cables that cost more than the HD650 on it, take out the foam disc covering the driver and even the back driver cover.
And all that still doesn't fix the bass. I wonder why there's no damping mods for the HD650 ... calling the son of markl ...