I remember a few years back I had the AKG 701 and bought the HD-650. I immediately disliked them due to vocals being a bit harder to hear. I just did not like how they sounded and I'm sorry I can't provide a better reason because it was so long ago. I wasn't as educated back then as I am now when it comes to sound quality.
Strangely enough I tried the HD-600 and immediately loved them. They are a good pair to go along with the AKG 701 I think!
There are a few reasons I went with the HD-600 despite not liking the HD-650. I know you had asked about the HD-650, but I think the HD-600 is worth checking into.
First, in a recent post someone mentioned that the HD-650 is a more laid back version of the HD-600 due to slightly rolled off upper frequencies (of the HD-650). There is also a bit less bass on the HD-600 then the HD-650. That was a huge plus for me. I'm very, very surprised that someone thinks the HD-650 doesn't have enough bass. The HD-600 to me has maybe 5% more then i'd like.
The HD-600 is bright, but never gets to the point where it is annoying. Some of the upper frequencies on the AKG 702 (like in some hard rock) can get very annoying to me. They almost never do on the HD-600 and that's why I like them.
Some people might think i'm crazy, but the AKG 701 and HD-600 have some similarities! I just find the HD-600 more of a toned down version of the AKG 701, but yet with a bit more bass, slightly less forward detail and a much smaller soundstage. Most people won't agree with me probably, but vocals sound much better to me on the HD-600.
I have a feeling that a lot of fans of the AKG 701 might like the HD-600, even more so then the HD-650!
