Pros: Good mid-bass, relaxing sound, removable cables, comfortable.
Cons: Lacks sub-bass impact, lacks excitement to some.
Here's my thoughts on the HD 600. During the review, I'll be throwing in the similarly priced HE-400 and the lower end HD 598 for comparison. Be warned, these are not direct comparisons. I have sold my HE-400 and HD 598, but still remember how they sounded on certain songs and if I don't feel comfortable about any statements towards them I simply won't post them.
Build Quality/Design/Comfort (3.75/5): These are really comfortable, they have a bit of clamping force but you will easily get used to them. They weighed a lot less then I expected and the pads are made up of very soft material. I would say the HD 598 is more comfortable because it has less clamping force and softer pads, but the HD 600 is still excellent in this category. The cables are removable but they aren't as thick and don't feel as sturdy as the HD 598's cable. These are plastic with some metal in them, the plastic doesn't feel very cheap, but to me they don't look or feel like a $400 headphone in your hands.
Lows (4/5): The bass has good power at the mid-bass area, but under 40hz is lacking a bit in impact. The bass is not the fastest, I noticed the HE-400 had quicker bass when I listened to upbeat Techno, like the song "E" by Drunkenmunky. I would say the HD 600's bass is adequate, it never wowed me with power and speed, but it didn't sound anemic or very slow either. It did well with Trap, rap, and dubstep showing good extension and moderate power but it doesn't exaggerate the bass which makes those genres enjoyable to some. Comparing the HE-400 I actually think the HD 600 has more perceivable bass in most songs, because of it's stronger mid-bass which you hear more in music, but at the same time the HE-400 will seem to have more bass if you play a song that relies heavily on sub-bass. Comparing the HD 598, I simply felt the HD 600 had just a touch more quantity wise.
Mids (5/5): The mid-range is less forward than the HD 598 making the 600 a bit easier to listen to without fatigue. The mid-range is smooth, vocals almost never seemed to be harsh or sound fake in any way. The HD 600's mid-range felt a bit more full than the HE-400, mainly in the upper mids. I felt the HE-400 was a bit recessed in that area, even with new velour pads.
Highs (4/5): The treble in this headphone lacks some detail and excitement. The treble is not veiled, but I couldn't say it is bright. The HD 600 has close to neutral treble that makes it very easy to listen to. Cymbals still have some sparkle and energy but it isn't overdone in any way, sibilance rarely occurs on well recorded tracks and when it comes it isn't heavily exaggerated. If you like a relaxed signature, this will probably be a 5/5 for you. If you want a more dynamic sound with excitement, you should probably look elsewhere. I think the treble was less bright and fatiguing versus the HD 598 and HE-400.
Soundstage (3.75/5): The soundstage has a good size to it but it doesn't always give you that out of your head sound with some headphones. This is kind of a good thing, because the intimacy the HD 600 gives you can be more exciting. Sometimes when I listened to my previously owned HD 598 and the Q701 I auditioned these headphones seemed to make some songs sound too far away, which made me lose excitment. The HD 598 has a larger sound-stage, not only in width but also in height. I listened to "Kitchen Sink , Boxcutters remix" by Amon Tobin and I could tell the song sounded smaller on the HD 600 than the 598.I can not comment on the HE-400 because I don't feel comfortable with my memory regarding soundstage on them.
Amping & Synergy: The HD 600 is a bit harder to drive than your average headphone but it scales incredibly well. It sounded good out of my phone, even better out of my Vintage Fisher 400 & JA-S55 stereo, and excellent out of my mediocre headphone amp (RA53b). Comparing the sound from out of my cellphone to the 30+ year old receivers or my headphone amp wasn't a night and day difference, but I noticed the treble was more relaxed and less sharp, plus the bass had increased power.
Final thoughts: The HD 600 is a great headphone that can handle almost all genres. It has a smooth and relaxing signature that I personally love. You will need to look elsewhere if you want a fun and more dynamic experience, but if you're looking for a easy to listen to headphone that won't make you too bored the HD 600 should be near the top of your list.