When i first got the Cayin iHA-6 and iDAC-6 combo and paired it with my HD800S, i wasn't too thrilled. Compared to my previous HDVD800, there was more detail present from the dac, but it came with some extra brightness and a general sense of being a bit overemphatic. It sounds thin, the soundstage is small and the treble is not well-integrated in the mix when starting from cold.
 
It's now a month later i'd the units have around 100 hours on them. What i find now, after regular use and long listening sessions, is that the combo warms up and peaks 6 hours in.
 
The difference in sound quality after the dac tubes have completely warmed up, is nothing short of amazing and revelatory. It's like a flower blooming and revealing it's inner essence. It truly shows you what this Cayin stack is all about. All it requires is a bit of patience.
 
After 6 hours the tubes provide the warmth and depth that makes the music full, natural and liquid. The soundstage opens up and positioning is improved and very convincing. The overall tone is buttery smooth natural with slam and dynamism and detailed highs, which are held in an iron grip and kept it in a straight line. The whole presentation is extremely balanced and i can't point to any sharp edges or any dullness. It's simply alive and groovy. 
 
I've previously owned the original HD800 the HDVD800, but then switched over to this setup. Even with the HD800S, i felt that i might need another headphone to compliment it. I don't have the need anymore. After discovering my setups full potential, it has made my HD800S a genre master. Something i thought i'd never say about this line of headphones.
 
This is the only setup i've been satisfied with over the years. It's my end game.
 
Nevertheless i'm in a financial situation where i have to sell it, since i'm moving out. I'll definitely pick up something similar when i'm settled in my new home. There's no way i'm going to have any setup without a pair of HD800S at least. 
 
I'm not saying the Cayin's are the only good option for your HD800/S, as there are certainly others, but at it's price, i consider this an exceptional value with high end performance.
 
It's no wonder that Cayin chose the HD800, when shooting pictures of their stack, as they both look and sound great together.
 

Decided to just scrap my initial and very brief comparison and replace it, by my overall not-too-technical impressions of the HD800S, after having used it for more than 6 months.
 
First my overall impression of the HD800 after having used it for two years:
 
I believe the HD800 is a great technical headphone, but without any mods, perfect recordings or careful and very expensive matching of gear, which only 5% of Head-Fi'ers can afford, it's sound signature is not going to be entirely balanced. Lacking bass and piercing highs are often mentioned by people who were not happy with their HD800. Some people will claim that through careful matching with gear, the sound signature can be flattened out and the headphones perfected. However, judging by all the impressions i've read and my own experiences, i've concluded that the headphone itself, does lean towards a more trebly presentation, which does mask the lows quite a bit. So, the headphone does have it's own sound signature, It's not all the dumb people that don't know how to run it properly.
 
Now for the HD800S:
 
First let me start by saying that when going from the HD800 to HD800S, you'll miss some "excitement" that was provided by the peaky treble of the HD800. This is noticeable in well recorded classical and orchestral music with a big group of performers. The HD800 might still have the edge here. It's the HD800 niche.

However, after having spent some time with the HD800S, you'll notice that when switching back to the HD800, that the sound takes on a slightly more metallic and thin tone with the treble more emphasized.
 
I even went as far as to leave them both for a while, and come back unbiased, with a fresh set of ears.
 
The conclusion was the same. The HD800S is the more neutral, more true to life, better balanced headphone.
 
I want to point out that the sound i'm hearing from the HD800S, is not the same as in my first quick comparison with the HD800, six months ago. The HD800S might have settled in, or i might have refined my taste through extended comparison with the HD800. Regardless, i feel that one must spend at least a 1-3 months with new gear, depending on use, to evaluate it properly.
 
The tweaks made to the HD800S are not minor and inconsequential in my opinion.
 
A few words from Sennheiser about the tweak:
 
"The enhanced sound reproduction of the HD 800 S is achieved through the addition of the innovative absorber technology that was pioneered in the Sennheiser IE 800 – a breakthrough that preserved the audibility of very high frequency sounds by eliminating a phenomenon known as the “masking effect”, where the human hear struggles to hear frequencies of sound when lower frequencies of a higher volume occur at the same time. By absorbing the energy of the resonance, Sennheiser’s patented absorber technology prevents any unwanted peaks and allows all frequency components – even the finest nuances – in the music material to become audible. This innovation was a key element in making the IE 800 the world’s best sounding in ear headphone, and in the HD 800 S it helps to bring even greater purity and precision."
 
In my experience, this is exactly what it does. WIth the HD800S i can enjoy the whole frequency range, without feeling that the emphasized treble is masking the bass. It's overall a better balanced headphone and much more enjoyable. 
 
It's what the HD800 should have been, when looking at it's predecessors, namely the HD600 and HD650. ​I have sold both my HD800 and HD650, as i feel the HD800S covers all needs by itself.
 
The HD800S provides an honest, and at the same time balanced sound signature, where everything can be heard and enjoyed equally.
 
5 stars. The HD800S is Sennheiser's true flagship headphone.