I own HE 400 for a while and this was a huge upgrade from my 10 year old beloved M50 and nice compliment to my HD 650.
Things that I learned over the time with ortho like HE 400 is that you really need to give it some time to burn in. Leave it over night playing everyday for several days let's say 100-150 hrs.
The sound will keep changing as the components works together, just like a well aged fine wine the longer the better..more refined.
During this process you'll hear lots of sibilance, thin and recessed mids, less intimate vocal, hollow to thinner bass etc, but it's all part of the process which Hopefully it will get Better.
After the burn in, you will find the sound has opened up and has more body. The bass feel tight and soild..the best bass they say out there and less sibilant.
They also say HE 400 is darker sound but bright on top which I agree.
Now, about amping HE 400 is not as complicated as I thought. It is fairly easy to drive like some people say here. It does need a good amount of power but not necessarily way more expensive amp.
I personally use Fiio E12 to drive this can out of X5 and it sounds good. Definitely have more energy and punch, also it makes it a little more forward and intimate which I like. E12 has a nice sub bass boost prob at 30hz slide down to 60 hz. This is a perfect compliment when listening to EDM and other genre that require bass boot. E12 bass boost doesn't bleed to the mids but it does add more body to the sound and it's punchy. Of course this not suitable for every song but it's there ready for you when you crave for it. There is nice review on E12 pairs with HE 400 here:
I bought the hype of replacing the pleather with velour but to me it makes it sound ..less warm, lighter and even more sibilance, didn't like it. What I did and to me it significantly improve the sound is placing one ply of tissue kleenex cut to match the size of the pad..and tuck them in under the round pleather. So now you have xtra thin layering on top of the original sheer. How does it sound ? To me I hear more body in the sound, vocal sounds more intimate and a bit warmer, it still retains the original huge sound stage and depth and ..less sibilance. Everything feel more "together".
I find HE 400 is a little choosy in terms of the sound source. It really adores a warmer side sound source DAC.
I have Marantz SR 7007 A/V receiver which has a clean and warm sound signature and HE 400 sounds absolutely delicious with it. An even sound across the Frq spec, nice full body and lush mids and highs..NO sibilance. Very enjoyable.
At this set up: Fiio X5> E12> HE 400
will provide more than enough power energy. It sounds very clean, transparent, good punch and dynamic, great sound stage and depth. But .. I always feel that something missing..I want more body and a bit more bass (without the bass boost on E12). In this set up you still occasionally hear some sibilance here and there depends on the recordings of course. But I wish it was gone.
While Fiio X5 >E12 > HD650 is much better combo in short.
Out of the blue I recently purchased another DAP Hidizs AP100. It has neutral sound and a bit warmer than X5. This DAP is equipped with 24 bit/196 Khz Cirrus Logic :CS4398 which is warmer sound.
In short when paired with HD 650 I wasn't impressed neither with M50. Weird!
I reached for my HE 400 and...Wow ! The sound is as good as with the Marantz.
I've never heard HE 400 sound so good coming off a DAP by it self or definitely better with E12.
The full body sound I've been craving for is there, the bass is definitely improved onto more textured, solid and rounded and also more separation. Mids are really good and more importantly NO more sibilance. Instead you get a creamy lush highs that is very pleasings to my ears.
If you see my other post I was putting AP100 for sale due to incompatibility with HD 650 and M50. But with HE 400 I just discovered a whole new sound and I decided to keep it.
I am not trying to brainwash everyone to buy this DAP but this this is only my personal experience re-dicovering my HE 400. I am sure that they are many ways other than this.