Just over one month has passed and I already like my 4XX over my 6XX, for listening to progressive rock and metal, jazz-fusion, and other instrumental music.
To my ears, they have the perfect amount of bass: strong and defined, yet never overwhelming. Percussion instruments, such as low toms and tympani, have a nice sustain to them on the 4XX that I truly had not noticed before with other headphones. I find cymbal sounds more appealing on the 4XX than on my 6XX. Loud sounds that bring my M50X to the point of distortion, are handled perfectly by the 4XX, while the M50X sounds artificially bright in comparison. Music also sounds more spacious on the 4XX, and I perceive more separation among instruments. Perhaps I am simply paying more attention, but I am also hearing (and, at times, feeling) synth and electronic sounds that I did not realize were there before.
This is my first pair of planar-magnetic headphones. An amp is definitely needed for the 4XX. With my phone, a decent volume is only reached when surpassing 80-90%, while I have never had to turn my headphone amp's volume dial over 8 or 9 o'clock to get sufficient volume on the 4XX. It appears that they leak more sound than other open-back headphones, to the point that my wife can hear music playing on my 4XX while in another room. It is almost as if I am wearing a pair of speakers on my head!
At first, I had doubts about getting the 4XX, mainly because I was concerned about the many reports of quality control issues, but I have had no problems so far.
I could not be happier with my purchase. The 4XX is my favorite Massdrop-made product so far, and, when it comes to headphones*, possibly the last purchase I will make in a while, as at this point I do not believe my sound experience can get significantly better without spending, literally, 3 to 6 times as much on (supposedly) better gear.
(* I have started my IEM collection, beginning with the Massdrop EDC3. Next stop: either the Pinnacle PX or the Planamic.)