Well, to start with all i will say is this... these are the best headphones I've ever heard. They are simply outstanding.
Build wise... a simple design, very light, very comfy. The headband does it's job, but it is slightly on the cheaper feel, as are the size adjustments. BUT it's still got a nice feel to it and the headband and adjustment work very well. They go over ear just perfect and I never once struggle with them falling off or slipping. Pads are very good. They were a bit warm at first but it passes very quickly.
SOUND... breathtaking. I've had a lot of headphones. It's a hobby of mine, that an IEMs. Close to 100 sets from budget £10 to mid fi £270. If anyone would like a comparison, i have or have had...
Philips Fidelio X1
Meze Classic 99
... and some cheaper sets.
Some of these are really great headphones. The set tho closest to the HE400s are the Senn HD650s, both in price and style. Both open ear full size.
The 400s has THE BEST mids I've ever heard. Silky smooth and VERY detailed at the same time. Should you purchase these as your first proper audiophile mid fi set or first Planar Magnetic, then be prepared for an abundance of nuances you've never heard, but never strident, or over use of treble emphasis. I'm a fan of the bigger mainstream bands. Queen especially. But I have quite a big taste. To give some examples
On certain Queen tracks I was hearing harmonies i just didn't know where there on songs I'd listened to hundreds and hundreds of times. They were coming from all angles in my head space. Instruments are SOOO natural and very organic sounding. Instruments and vocals are so easily picked out and separate without ever losing the feel and shape of the song. Vocals in particular are a real highlight. Karen Carpenter was simply gorgeous to listen to. Silky and effortless in her delivery. Beatles had weird and wonderful sounds coming from all over the place. These headphones are so much fun to just lie back and listen to. You'd think the way I'm describing these they are overusing treble like a lot of other audiophile sets do to make them sound super detailed and sparkly, but they really don't as they are really smooth and never fatiguing. Easy listening but if you really wanted to you could set out to hear new parts of your old favourites by giving them a more closer, detective like listen. Every instrument just sounds wonderful with these cans.
Soundstage - these are very 3D like. Very wide stereo effect, but they have great layering and separation, great depth and height, forward and back.
You'll hear a lot of people say they are bass light and you need to use Focal Pds to get the bass to come out. Don't let this fool you in to thinking the bass is not there. bass notes are still clear, they are quick and never bloomy. Great resolution. The just don't have that slam, that deep sub bass rumble. But neither they should as the 400s is not that type of headphones, they are going for balance. I'd never use them for bass heavy EDM or electronic music (although I'd guess they still do these genres justice). i have good closed back sets that i can swap over for these types of music. I never feel the 400s cheat me out of bass. They do something very clever with a slightly enhanced mid bass that still gives them enough punchy anyway. These aren't like Philips SHP9500, a set that really does struggle with bass.
The 400s sound great from any source, but like most headphones they do even better AMP'd. This doesn't have to mean you spend a lot of money on a DAC or headphone AMP to power them. I use a simple set-up of a Sabaj D3 DAC and Topping NX2 AMP. The NX2 especially gives them a little more neutrality, without losing their overall smoothness, but it helps bring out that separation and clarity even more than say something like a FIIO AMP (like e10k) or a tube AMP which are slightly warmer and thicker sounding amplifiers. That's not to say tho they are second best, it is just my own preference. Everyone has their own sources they like. I'd say phones will drive them fine to, but if you'd like a wee bit more power the NX2 or a Fiio A1 would help. Both cheap.
OPEN BACK - Remember these are for home use mostly. People round you WILL hear your music and may see that as a distraction or annoyance. These are for sitting back in your room and appreciating what they do to your music collection or music enjoyment. Leaking sound tho is what all open ear sets do. At high volumes in your room I never disturb or hear the music from outside in the hall or in the next room. I will say this, at a more reasonable volume level these would be fine in an office environment IMO as leakage isn't annoying or loud. It's there, but more polite and less obtrusive. Any you'll still hear your music fine and won't miss out or strain to hear your songs.
TV/Gaming - These are great for movies due to how much sound they pick out. I watched Thor Ragnarok and the action was immense. Sounds and effects coming from all over and outside your head. It makes voices so natural as if they are talking to you personally. Gamers will appreciate these for hearing enemies coming behind you, you'll hear their foot steps.
400s v hd650 - The 400s are better IMO. The hd650 require further expense as you'll need a good AMP to run them to ge the best out of them. Probably around £100 minimum. The HD650 properly AMPd sound great, but the 400s sounds clearer and less veiled. HD650 has better bass, but the 400s never misses out on notes. Mids go to 400s every time.
All I can say in conclusion is if you are venturing up the scale of headphones. If you want something that brings out micro details in a non fatiguing pleasant manner, if you want to hear vocals how they should be heard as if you are in the same room... buy the 400s.
EDIT : Sadly have to knock 2 stars off as the sound on my set became heavily distorted and crackly. Couldn't put it up to more than 65-70% volume before the sound giving out on certain tracks. My predicament is buying a new set as my refund has been issued. I really do love the sound, but this is the 2nd HiFiMan product to break on me after the Re-400 IEM did some years ago (after 5 months). I's give it 5 stars for sound.
EDIT 2 : Bought a new set, couldn't resist price of £180 NEW. Hopefully will last longer than last set.