In the very old days, I compared very unscientifically both the HD600 and HD650 at a mass-market-ish store. The 650 was full and bloomy, whereas I found the 600 to be thin in the midrange, and a bit grainy in the treble. None of this is relevant today, noting that at least the 650 has been revised, and the store used a rather poor source.
But, today, I have a 5-year-old 650, and I would buy it again. I do have to find amps that are flat so that the midrange fullness (not major) is not overdone. But the Phonak PFE 232, which is thin in the midrange, analogous to the 600s long ago, requires a plump midrange to make them fun and realistic.
But again, listen to new stock and consider the equipment interactions.
Meanwhile: I may have, a week from now, my new balanced Cardas Clear headphone cable, to use with my RSA SR-71B amp. The latter sounds great with my Cardas Clear Light unbalanced cable, just great and quite livable. But we'll see what they heavier cable (twice the copper) and balanced output (unbalanced input, though) sounds like.
(PS: I stick with Cardas because, as in my home system, they get "flat, balanced, and neutral" while "revealing" just right, increasingly so with the latest versions, using technological innovations that make sense to me (not a novice in acoustics and engineering....) and make sense upon listening, with prices that aren't "what the market will bear" but which match "the cost cost of doing business". I'm not fond of ultra-detail at the loss of body and neutrality that, at least in the past, some silver cables have sounded, nor am I as comfortable gambling on a young company that buys wires already made and merely assembles. Having said that, my recent Moon Cu interconnect is pretty good.)
BTW: I'm now a major fan of the RSA Blackbird, latest incarnation: balance of all the right virtues, at least with the HD650. I got it as both home and mobile amp, because I don't think I'd find better balanced quality anywhere near the price. And I don't want to use up space in my usual listening chair. PS: I'll take it to the San Francisco meet at test these all with all sorts of sources and amps, and share them for listening among the gang....
(Lately, my health is better, so my energy should be better by the show date. Assuming nothing nasty shows up in my next MRI and my next PET/CT scan. I'm may be glowing with re-radiation by then....)
Edited by Stoney - 7/14/13 at 10:16pm