Pros: Very detailed. Balanced. Good soundstage. Absolutely amazing mids. Entirely modular, with all parts replaceable.
Cons: Bass could be a little tighter. Some would say subdued treble, but I'd say it's at a very realistic level. Soundstaging has room for improvement.
Note: The price paid is used from Amazon Marketplace. Also, I've duplicated my review from the other product listing here on Head-Fi. If it is possible, the two HD 600 pages ought to be merged.
The first thing about these the HD 600 is the excellent tonal balance - it's never harsh, but the highs are still there and very detailed. Sometimes the finest details may get lost in very complex music thanks to being overshadowed by other parts of the music, but the resulting detailed smoothness is a very good thing.
The mids - ahh, they're to die for! They're the highlight of the headphones. They're detailed and lifelike, similar to the highs. But they're so realistic sounding - even so more than the HD 800 and Beyer T1 I've auditioned. As far as I'm concerned, the HD 800 (haven't heard the HD 650) still falls short here in comparison. Guitars, brass, saxes, vocals, and more sound so much more realistic (and and have more visceral impact) than anything else I've heard (my new Alessandro MS1is come close, but not as much with the low mids).
Bass is fairly tight and at just about the right level for all types of music. You'll never make anyone happy with any given quantity of bass, but I think these are as close to a neutral level as you'll get. Very low bass can be a little loose, and rapid bass notes aren't as distinct as the HD 800, Beyer T1 or DT 880, but it's still excellent overall (and especially compared to lesser headphones).
Their soundstaging is very good - but there are, of course, far superior headphones for this. Depth is good, width is good, and precision of placement is good - again, a very balanced sound. The sense of spaciousness (a function of all three factors) falls a little short of the AKG K701, and of course the HD 800 and Beyer T1 - but of course is far more evident than the close-in Grado sound. The "It sounds like they're playing in the room with me!" sensation never really happens to the same level that my speakers (Infinity Renaissance 90) can provide.
The build quality is very good - the headband and frames are supposedly made of carbon fiber, but after inspecting them (and seeing photos of cracked headbands) I believe it is probably a carbon fiber reinforced plastic instead. Less impressive to your friends, perhaps, but an excellent material nonetheless. They do of course feel slightly plasticky as a result (but it never feels like cheap plastic).
They're quite lightweight, and the padding is excellent - the velour earpads and headband are very comfy. I know some complain about them being too tight - I can't really comment, since I bought mine used. I never found them to be too tight myself.
They stand up to abuse admirably well - mine have lived in my backpack for four months straight and only have superficial damage to the grills (they get dented when sharing the backpack with my Nikon) to show for it. They haven't been babied at all - just no abuse like trying to bend the headband over on itself... Anyway, since everything is modular, you can easily replace any part that has broken. I just got new pads and a cable to replace the worn originals.
The marbled finish does wear off a little over time, but it has yet to become detrimental to their appearance so far. Oh, and by the way, the finish looks a whole lot cooler in person than in photos of the headphones.
I've found amp'ing them not to be as difficult as some seem to think. A/B'ing my uDAC (my main source and amp) against a Beyerdynamic A1 amp, I really couldn't consistently tell any difference between them other than ultimate power output. The Sansa Clip+ actually does an admirable job running them up to moderately loud levels (not short-term ear blasting, of course), and never clips with them when playing music. It does, however, sound slightly (yet noticeably) less dynamic and with a narrower, less well defined soundstage in comparison to the uDAC. I forgot that I have a crossfeed plugin (HeadPlug MKII) running on Winamp, so I have to retract those opinions for now - although it does stand for now that I can't get a crossfeed with the Clip (without using an amp with one).
So, the verdict? These are amazing headphones, deserving of their place as one of the best dynamic headphones available to this day (15 years after their first introduction!). Their smooth, slightly warm (not dark!), but very realistic and - yes - exciting yet neutral sound is near-perfect in balance. Soundstaging is perhaps the area I would like to see the most improvement, but it is good in this respect already.
You'll have to pry them out of my cold, dead hands.