Nice to see these headphones getting some love here. I saw this thread earlier this evening and decided to throw on my pair. It has been a few months and I wanted to see if I still loved them. I have moved twice in the last 9 months and just recently had some DAC issues so it is a bit complicated, but I am very confident I have a handle on these things and will let you know what I think about how they compare to HD600 from memory and a few other higher end phones that while not a fair comparison may be helpful to others none the less.
My system has changed significantly over the last year so I will start at the beginning with the R70x... heard them at a meet in Minneapolis, immediately liked them better than my HD600 after listening to them both side by side on my Gustard stack (X12/H10). I will stand by that judgement call to this day, but after more listening on a few more different systems I would not be so quick to assume everyone else would come to the same conclusion. The HD600 and especially HD650 have a richer more full bodied sound and IMHO more cohesive and balanced across the range for a wider variety of music. If you share my love for the Audio-Technica house sound however, and sometimes feel like the Senns can be too smooth at times, then yes, the R70x is a real contender. First, it has a little more magic with my favorite vocals, mostly female, but a better midrange in general most of the time (but not all the time). If find that brass has a little more realistic bite without getting harsh, and as others have mentioned I think it delivers a faster and better delineated (more instrument separation) presentation. Bass is not as deep, and has a little less punch, but I think it has better texture and definition and thus more enjoyable for me. I ordered a balanced cable from Surf and plugged them into my Cavalli Liquid Carbon v2 and was completely blown away. I am not sure how I feel about balanced anything, but in this particular case the synergy was there, bringing the whole signal chain together in a fantastic way.
Downside? Sure, I think the imaging is not as wide as it could/should be, and can at times get a bit congested sounding when the going gets tough. Depending on recording, the bass can drift into a bit of a wooly direction, losing some clarity around the edges. Treble is to my liking, but not as consistent and extended as it could be, and a little rolled off in general (which to me is correct, I am an AQ Nighthawk fan so I think most phones are bright by default).
My real reason for keeping them while others have come and gone however boils down to weight and comfort. When I want to just do some casual listening and not worry about my ears getting hot, my head feeling pressurized or my neck feeling burdened, these are my go to "relaxation" headphones. Not to take away from what I feel is great performance for the buck, but at the end of the day it is more physical than musical merits for me. I listened today on my latest system setup Emotiva DC-1 DAC (waiting to get my Matrix X-Sabre Pro serviced) feeding Cavalli Liquid Crimson (Carbon was a gateway drug, my wallet is still crying). A bit overkill for this headphone, but at least I feel like I am pushing it well beyond it's full capabilities. I do think it needs a good kick to come alive, even if you are experiencing good volume you are probably missing out just a bit on the added grip and control a strong desktop amp can give it.
I am getting to the point of sending these back out into the world as my needs have changed, but before I do I wanted to stop in here and pay my respects. I think this is a great headphone. Compared to AQ NightHawk, bass is not as deep but the upper bass and lower mids feel a bit better defined. Top end is very similar and of course the voicing is different. I think the AQ are more honest and consistent though they don't sound as "flat" as the R70x can sound on some music. Ditto the Beyer DT1990, better bass and even a smidge more resolution across the board than both the AQ and AT. If I wanted something to do double duty, both casual listening and some headphones for a home studio or serious musician, the R70x is a great starting point but the DT 1990 does bring you up to another level (at twice the price).
For the last year my game plan has been R70x for fun, DT1990 for bigger punch, extension and dynamics when I am in the mood, and something close to end game TBD for me as the personal flagship. Well, I just got lucky and picked up a new HD800S, and it is so comfortable that the R70x is just not going to get the head time it deserves. As I was mostly swimming around at the mid-fi level (hope that doesn't sound pompous) for many years now, I really wish I had spent that time with the R70x instead of the many others I had come through my house, but they just didn't exist back then. Yes, it is not the best studio tool, and it is not really all that influenced by the AT house sound, but it is there... but if you value definition and speed and a little bit of that airy sound AT vocal presentation these are a real gem. I have recently switched to Martin Logan ESL (the entry model) for my small living room, and I think the voicing between those and the R70x is actually very similar. Small, light, quick, and a dangerously charming voice. What's not to like? If you want more extension and resolution without giving up any of those strengths, you are going to have to really cough up a lot more bucks.