A layman's review of the HE400se also a little heads up on things i found out after using the headphones

Full TL;DR-The only amazing thing is their sound quality(id give 6damn stars if I could) and comfort, everything else is sub-par or shit

1) They will sound terrible on mobile phones , even flagships like the samsung galaxy note10 , the macbook air 2017 is also not enough for youtube/spotify, if you have a gaming laptop it might just be able to power these(Youtube/Spotify will still sound a bit underwhelming)

TL;DR: Get a DAC+AMP-Shanling UA2, Fiio BTR5,Fiio E10k(8year old model) are some relatively cheap recommendations

2)They will sound "bad" right out of the box , burn is a real thing and these require some burn in (mine are approx 25hr-burn in done) they will get a bit louder , more clear , spacious and accurate the more you listen to them

TL;DR: Burn in is required

3)Another reviewer said the build quality is bad , he is right it is shit , cheap earcups(not earpads, mind you) , grills and what stands out the most is the cable , in Hindi we have a word "Tangna" , its a metallic wire used to dry clothes , the cable is basically that , but thinner and there are 4 of them, its an absolute joke. Also they are very fragile, their weight does not match with the build quality DO NOT DROP THEM. Do not press them hard on your head, it will sound like crackling chocolate wrapper and that is because the diaphragm is as thin as a plastic wrapper and fragile so , it might tear the diaphragm, they're comfortable though, nice earpads but they will heat your ears during the summers

TL;DR: Build quality not the best + they're fragile

4) THEY ARE OPEN BACK they leak a LOT of sound ,and dont isolate at all , even at 10 volume they leak enough sound that its audible by people in the room .Also they're full size headphones so they are humongous, by normal headphone standards.
TL;DR: Thyre not portable headphones

5)The packaging is meh , but one good thing is the number of authenticity seals , there should be 2(top and bottom) on the headphone box and one on the cardboard box.

Sound quality review:
go to headfi.org , they have more expertise than me in terms of sound ,
but,
They sound phenomenal absolutely phenomenal , hence the 5stars, but one thing though you will not "feel" the bass you will "hear" it.

Shipping/Customer service :
It was impressive the headphones arrived in 5 days(3 Business days) to Delhi , no hiccups whatsoever .

I'll preface the review by saying that this pair sounds like a completely different set of headphones after burning in. When I first got them I was not at all happy with the performance but I held off on the review for about 20 hours of listening and boy was it worth. With every passing hour these got better, and by the time I hit the 20 hour mark, the headphones exceeded my expectations.

Physically, the headphones are built really solid. The earcups are as comfortable as can be. You can listen to it for hours without a hitch. Putting them on for the first time, they feel underwhelming. It takes some time to get going but once it does the sound is amazing. The mids are crisp, the bass is just the right amount of punch and the soundstage is extremely wide. The imaging is so good that you can almost imagine yourself sitting in a theatre listening to Beethoven. Adding to that, the headphones are pretty easy to drive. I have reasonably high end laptop and phone. Both of them could drive this pair easily. Although I'm looking forward to a DAC to further improve the experience (have my eyes on the Zen Dac V2).

Unfortunately these headphones aren't without flaw. The accompanying wires are absolutely abysmal but fortunately they can be easily replaced. Another piece of equipment I have my eyes on. And finally Hifiman infamous for its QC did mess up one thing. The left earcups are just a tad bit quieter than the right. Initially I thought it was just me but everyone who tried this pair agreed. Although easily corrected with a little EQ balancing, it's not something one excepts from a new pair of headphones. This might also be rectified with the new set of wires but until then any device without a balancer can't be used with these headphones.

Awesome packing and delivery experience!

Overall, HD599 is better.

Bass is excellent and neutral.
Soundstage and imaging is average. HD599 is slightly better.
Treble is very bright. Need EQ.
These are very resolving and analytical.

Biggest issue is the foul smell. Comes from foam pads.

This is my first experience with open back (or planar) headphone. Note that this is the international 'Stealth Magnet' edition.
The sound quality is good - bass is controlled and not muddy (though there is not too much of it). Mids are good. Treble is also nice.
The cable is horrendous though - not so much in terms of sound quality , but it is stiff and has quite a bit of microphonics if you move.
One thing I was quite disappointed about though was sounstsge, or the lack of it. I was especting a wide soundstage with atleast some depth. Unfortunately, it still seems that sound is coming from within the head rather than outside. Similarly, I did not find imaging to be any more accurate or different from a good closed back headphone. I have tried driving it from mobile and two different laptops. I will report back if a dedicated DAC/Amp changes that.
Comfort wise, the pads are nice and comfortable. The headband will take a whole to break in as it still feels uncomfortable when listening for more than an hour...
Buying experience and delivery was fast from Headphonezone.

Gave them 80 hrs of burn-in and these are so fun to listen to. Sure my DT990 pro are way more resolving and analytical but these are just buttery smooth and super fun to listen to, great soundstage, dopest of the comforts and cleanest of the bass at the price.

One minor gripe, that cable is hideous; like really Hifiman?

I mean cheap KZ earphones have better cable, I never expected myself praising that KZ cable, but these are just that bad.

You should opt for a Cable replacement. I have balanced amps so its all good for me, but if you don't have a balanced amp, you need to buy adapter cause replacments are only for balanced cables 🥲🥲. Invest in ddHifi adapters, small, sturdy and good looking.

Also also, these are not gonna work without a dedicated amp, these need power. Some phones at max volume might push some amount of volume through this, but you will be left for wanting more.

I suggest go with BTR5, have a balanced output, can power these(atleast to my satisfaction) and makes these portable.

For desktop, ifi Zen Dac V2 works wonders.

My left channel stopped working on HE400se, apparently this happened on my first order as well but Headphonezone got this replaced since it was under warranty. But now my left channel stopped working again and it just crossed the warranty period. I should have realized the first time when the left channel created the issue and should have avoided it.

Build:
They look better in flesh than they look in the pics, the overall build feels solid, the pads are pretty comfortable and they don't feel heavy on the head.

Sound:
I'll keep this short as there are many reviews available online.
-The resolution is on-par in this price range.
-Highs are smooth and don't get harsh even on higher volumes
-Vocals sound clear.
-Soundstage is wide enough but not super wide.
-Bass is punchy and clean.

My Setup:
I paired these with a Fiio BTR5 using a Meze 99 Series OFC 2.5mm Balanced Upgrade Cable and they sound great on low gain, can always switch to high gain for extra punch.

Do note that these do require a decent amount of amplification to sound loud enough.

These headphones sound good, they are sharp and fast. But, the overall experience is tainted by the worst possible cable in market. I use them with a BTR5 and am looking forward to changing the cable ASAP.