I got the Hifiman 4xx one week ago, here my thoughts and some comparison against the Sennheiser hd600: Build quality: Good. I got some minor imperfection on the plastic headband part (where the Hifiman logo is) basically I can see some yellow glue on the right side (on the left side is not present) and the plastic part is not perfectly attached on the headband. For now, it's not an issue. I hope it will not detach in the future. Comfort: Excellent. They are heavier than my Sennheiser hd600 and equally comfortable. The earpads are very plush, like pillows. Headband makes a good work to distribute the weight. I got other headphones with less weight and much less comfortable than the 4xx. Massdrop and Hifiman did a great job on this part. Power Requirements: Not hungry beast. They are very well driven by my Audioquest Dragonfly Red (2,1V Output) and they got the same volume as my hd600. I can even drive them with my MacBook Air headphone out, not the best pairing of course. I will say even a basic dac/amp can drive them with good results. Another good point. Sound Quality: Amazing for the price. I am still burning-in (about 50 hours now) But they sound as good as my Sennheiser hd600, and they beat them in some areas. Bass is full and extended and more visceral on the 4xx. Easy win here for the planar driver. Also, there is no mid-bass hump, so the transition between bass and midrange is more linear. Midrange and vocal performance are nice on the 4xx, but nicer on the hd600 that got a very special midrange with some upper-mids emphasis, vocals and guitars cut in the mix. The 4xx has a more safe approach, upper mids are more controlled and pushed a bit down. Compared to hd600 the 4xx can sound a bit muffled in the presence region, but honestly, I will not call the 4xx a muffled headphone, it's very clear sounding just missing some decibel or two of presence, where the hd600 is a bit too forward. Treble is smooth and controlled on the hd600, a bit more forward sounding on the 4xx. Very resolving on both headphone, maybe the 4xx just a bit more. Soundstage and instrument separation is where the 4xx easily outclass the hd600. The sound is more wide and open on the 4xx, big difference. The background is blacker with the 4xx planar driver and you can pinpoint instruments easily. The hd600 sounds more intimate, like switching from a bigger room to a small one. Instruments are closer to each other and sometimes a bit more confused sounding. Both headphones are tuned very well and they sound close to what I consider neutral/natural sound. The difference: 4xx has more low-bass and upper-treble emphasis, the hd600 more mid-bass and upper-mids/low-treble emphasis. I am pretty impressed by the 4xx. I was looking for my first planar and the 4xx deliver. I think the musical presentation is better on the 4xx. The combination of big soundstage, black background and instrument separation is very easy to like. WOW factor is here. I'll never sell the hd600, for that midrange I love. Vocals and rock guitars really cuts, an absolutely classic headphone. EDM, Dubstep, Electronica the 4xx has the edge. Rock/Metal is a draw, The 4xx has better speed and better impact, the hd600 better guitar tone. Acoustic music is better on the hd600, 4xx is very good too. Overall... It's somewhat a draw. Hope it helps!