My experience with the HIFIMAN HE-4XX has been great so far. I've had them for several weeks now. Mine arrived a week before the expected arrival date. Also, mine has been operating as expected with no physical defects. So far no problems with connections, grills, or yoke screws pulling out. The chunky build certainly makes these headphones look like a piece of audiophile gear as opposed to mass market or pro-style headphones. Overall they are comfortable to wear with a light clamp though you can feel the weight. No hotspots on the headband. The earpads are deep, mostly soft and comfortable though there is a slight scratchiness due to the material that touches your skin. I would go a couple hours wearing them but probably not much more than that.
Since I'm used to having subwoofers as part of my audio systems in the car and home, and I still manage to get out once or twice a month to the nightclubs, my initial impression was that the HE-4XX were slightly lacking in bass volume; though the quality was excellent; tight, controlled, and extended deep. After listening to these critically for some time though, I came to realize that I was actually hearing more accurate bass at a more natural level than I am accustomed to. These seem to produce a lot of detail, control, and speed throughout its range. There's nothing soft or sloppy about these. They are bright and lively, perhaps a little dry but still manage to have a lushness to them that I might normally associate with a warmer sound; but these are not warm nor dark. Perhaps much of what I'm hearing that is so good is imparted by the planar magnetic technology as this is my first experience with this type of driver.
It is also my first experience with open headphones. While the soundstage is wider than with my earbuds and closed headphones; It's nothing like the soundstage you get from in-room speakers. Separation of musical instruments and vocals were great though.
Overall, these headphones don't as much make me want to get up and dance, thought they still impart some amount of motion on me. Mostly they make me want to sit down, listen, and enjoy the experience. The sound is more like being at a concert as opposed to being at a nightclub. I want to go back through my entire CD collection and listen to it like it is the first time again. After a couple hours of listening though it does become time to put them away; at that point the physical comfort wears thin but also the detailed, bright, and punchy character of the sound, just as with a live rock concert, can get fatiguing.
I think a reasonable DAC and amp is really needed. I listen through an economically minded FX Audio DAC-X6 which is enough to get me there. I did try to listen only using the computer's on-board audio (Realtek ALC 889) with an SMSL SAP-II amp, only to find some music to be difficult to listen to for very long due to a harshness on the top-end. Garbage in, garbage out; accurate audio gear tends not to be very forgiving of poor source material and lackluster upstream components (the SAP-II is fine, it is probably the combination of the SAP-II and HE-4XX exposing the limitations of the computer's onboard audio implementation).
I find the HE-4XX to be awesome audiophile headphones and I love having the option to listen to music on these when I want an immersive concert experience without cranking up my in-room stereo, or want to do some really critical listening. Also there are times when I'll need the open headphones so I'm not totally isolated from the goings-on in the house! As my daily-driver for gaming, for longer sessions, for when I need isolation, or for when I want nightclub-in-your-head sound, I'll still go to my Beyerdynamic Custom Studio headphones first (they are like comfort food LOL). But honestly, I do think that the HE-4XX provides a more refined, accurate and truer to the original recording sound than the Beyerdynamic. I'm really happy to own both of these headphones, and it would be a terrible day i I could only keep one of them!