Want to add my $.02 to this thread....
Background
I'm a total newbie to the realm of 'good' headphones. I'm not new to the experience of good sound, as I've enjoyed good home theater sound for years. My current home theater conists of Axiom and SVS speakers, Emotiva amps, and Pioneer Elite sources. It is, IMHO, a great sounding system for a modest amount of money. While I have not had the luxury of owning uber-buck stereo equipment, I have a few friends that do own $50k+ stereo setups (yes, stereo; no TV, no surround sound, just 2 channels) and so I know what audiophile level systems sound like. I've also spent some time with a $150k+ Dynaudio system.
Choices
I decided I wanted a semi-portable rig. My recent headphone experience up to this point has been a pair of Sennheiser PX100's in my office, a pair of Shure E3C's for mowing the lawn or working out, and my wife's pair of Shure E2C's. I decided to purchase the AKG 701's and HD650's, based on discussions here and elsewhere. Yes, I realize there are a half-dozen other models I could/should have considered, but I didn't. For Jazz + Classical music, these two cans were most often mentioned as 'the best'. I don't feel that I could justify the cost of keeping both pairs, so one is being returned to the store. I knew I'd need amplification, and so I went the budget route and picked up a PA2V2 and a Total Bithead. I know these are not considered the 'proper' caliber of amp to drive these headphones, but it's a start. I also knew I'd be using my iPod as a source from time to time, and so I picked up an ALO Bling Bling dock.
Setup
I know the AKG's infamously need a lot of break-in time, and so I played them for almost 2 weeks straight, repeating through my iTunes playlist. I do think they sound a litle different now, with a touch more bass and possibly, ever-so-slightly calmed down highs.
Conclusion
So here's the deal. I've been playing with these headphones now for a couple of weeks. Switching amps, switching sources (PC vs SACD vs iPod), and switching cans. Regardless of the 'it's too veiled' and 'too warm' arguments, to my ears, the HD650's are the keepers.
Thoughts
Testing has been tough. Both cans are superb. While I dig the retro-chic look of the AKG's better than the plain-jane HD650's, I can't escape the fact that the HD650's are just more pleasing to listen to. When I'm listening to the HD650's, I'm carried away on the music. I forget that I'm listening to headphones. The world melts away. I'm not too macho to admit that the HD's have brought a tear to my eye on certain songs. The AKG's don't do that. When listening to the AKG's, I find myself more involved with the technicalities of the recording. I fully appreciate and understand that the AKG's are probably more revealing headphones. They demand high-end components, perfect sources, and a critical ear to enjoy them. I won't disagree that they need better amplification than I have. They are not for the faint-of-heart, as they will render every 128kb MP3 as a whimpering mess of compression artifacts. They will also reveal every microscopic detail in a recording and thrust it upon center stage and force you to take notice. I've come to my own conclusion that unrelenting detail is not the be-all, end-all of headphone superiority.
The HD650's don't thrust anything on you, they just present wonderfully rich and velvety sound. The AKG's are fantastic for picking up the sound of a mouse burping backstage, and some people like that. The HD650's are fantastic for making the music sound real, full, and lush. All of those details that the AKG forces you to notice are clearly present with the HD's, they're just not thrust upon you. I am extremely impressed at how good the 650's sound, even with these 'mediocre' amps. I can plainly hear the difference between the iPod alone vs. iPod + amp, and so I suspect they would benefit even further from a better amp. But that's the rub, I don't think they absolutely require an expensive amp, sources, cables, etc. And that's why I'm keeping them and returning the AKG's.
With these 650's, I can simply sit back, grab a cold beverage, and enjoy my own personal concert. The 650's give me a musical-emotional experience more similar to that of the uber-buck stereo systems that I've experienced. And that, in my book, dictates the best headphone for me.