I simply adore my HD650's.
I bought both AKG701's and the HD650's last year and spent a couple of weeks listening to both. I also bought a handful of headphone amps too, as I knew that both headphones really needed amplification. And they both do, no question.
The AKG701's are more clinical; good bass, clean midrange, but nearly too-bright highs for me. They are absolutely lovely headphones to behold - they're built well and look 'expensive'. The HD650's are different. They don't look all that special. Just like big black headphones. Their sound is more 'laid back', but not too much. Strong and powerful bass, beautifully warm midrange (but not too warm), and clean controlled highs. The real test came with music that involved rich & detailed midrange stuff - Diana Krall's voice, YoYo Ma's cello, etc. On those, the HD really shined over the AKG's. Stunningly beautiful.
The AKG's were fantastic headphones, but they weren't quite the 'nth' degree of amazing (to me). I would have been quite happy to keep the AKG's, but the HD's just did it for me. I'm also a fan of B&W speakers, which are a tad too warm for some. So it is entirely likely that the HD's just meshed better with my brain. YMMV, of course.
So I returned the AKG's and kept the HD's. I have mine powered by a Little Dot MKII Tube amp. It is my office setup, and I dare say that it rivals the sonic bliss of my M80v2 setup at home.
And the others are absolutely correct about open-back headphones. Most 'audiophile quality' headphones are open-back. Which means you can more-or-less hear everything around you while you're wearing them. And the sound from the headphones escapes into the surrounding environment as well. If the goal here is for you to be able to pulverize your eardrums while your significant other sits at the other end of the couch in sonic ignorance, you need to look for sealed headphones. Otherwise, they will hear your music. HD650's, AKG701's, and all Grados (I think) are open designs.
If you're looking for sources for headphones, check out HeadRoom
. They are a great company to deal with and their prices are usually competitive. All they do are headphones & accessories, so they know their stuff. They will stand behind their 30-day return policy and will help you with questions or problems, unlike what can happen with Fleabay. Feel free to email them and describe what you want to do, and they will most certainly recommend certain models that will fit your budget & desires. And no, I don't have any relationship with them other than being a satisfied customer.
Good luck! And don't spend too much time over on Head-fi. That place will suck your wallet dry.