I will try to add a few more words about HEDDphone TWO after my first 4 days of listening. I hope I will not start some useless debates here and I ask everyone to consider these as my honest personal opinions. Since it is a personal view, please take it with a grain of salt. I am also not a good describer of what I hear, but I'll do my best to write something coherent. I feel there is not enough information available about a very interesting design and these headphones deserve to speak more about them.
About signature:
I honestly think that HP2 has no signature, it is just transparent and sounds as the music has to be heard from the production desk. The main purpose of these headphones is not to enjoy music but to hear it as detailed and clear as possible. In short words: to work with music, not to listen to it. This is why most people complain about the unexciting nature of it. It's true, they are really unexciting because they lack a signature, but you can add one with smart pairing or equalization. They are also not flat on the whole frequency range and this is, most probably, a limitation of the current version of the AMT driver which reproduces in the best way the midrage but has some problems on both extremes.
About amplifier and cable pairings:
Here I need to describe a little bit how I listened to these in the last 4 days, giving them around 40 hours. Yes, I was quite addicted to them. Of course, my opinions are preliminary and might change. However, even having low hours I consider that I reached a quite stable phase where I think I can share some thoughts.
In the first hours I have used the stock cable and connected the HP2 to my tube amplifier. My tube amplifier is tuned neutral and the signature of the manufacturer was a wide soundstage without harshness, but polite on the top end (this can be also a disadvantage but in case of HP2 avoids any sibilance on top end).
After listening to a few of my favourite albums I was disappointed. Everything sounded flat, unexciting, there was no "wow" factor. For me both highs and lows seemed recessed while the midrange, even if it was very detailed, was dry and sterile. Another problem was that I had big problems to align my expectations to the presentation of the headphone. And here my problem was somehow what I call the sound distribution (note that I avoid using the commonly used "soundstage" term) because in my view, while listening for the first time to an AMT driver, the biggest difference was that the sound was somehow distributed differently than on the headphones. Lacking impact and that "visceral" sound you expect to have from a headphone, which puts the sound more or less "in your head", the HP2 was more a kind of speaker-like presentation where the sound was somehow floating around you and the details were in other places than you were normally expecting to find. Of course, as I said, it might be a problem of mine being unused to AMT drivers or just placebo because my brain was somehow associating their look with tweeter speakers.
In order to mitigate what I was not liking to the sound I switched, after a few hours, to my Forza AudioWorks Noir Hybrid cable and immediately it added more body and extension on both ends. Highs opened as mentioned by several people as being a consequence of burn in. It was an idea warmer sound, more "wet" and lively, with more bass quantity (do not expect here sub-bass wonders but more quantity while not loosing articulation) and the highs went an idea up. So for me changing the cable was a very audible improvement and a big step towards what I was used to like in a headphone. Using this cable for more than around 20 hours did not help me identify any further improvements, which made me think that the headphone drivers do not evolve in time and the jump in sound quality was related mostly to the replaced cable which already had a few thousand of hours on it.
After this phase I took the decision to listen to these headphones on my TT2 integrated headphone amplifier, a recommended pairing by many. While I normally listen to heavy metal and the midrange is the most important aspect for me, I decided to go this time for something more orchestal with female vocals. Because my FAW Noir Hybrid has XLR connector, this forced me to return to the stock cable with the 6.3mm jack. I felt immediately that I lost some bass and highs, the sound became more flat and some of the female voices were sibilant in specific passages. Not everywhere, but here and there. While the clarity was exceptional, the midrange was an idea too dry for my taste. So I was back where it all started, this time with a brighter signature because of the TT2. I did not felt that there were any power or amplification related issues between TT2 and my tube amp, even if TT2 is much more powerful. So in my view, HP2 does not necessarily need a very powerful amplifier, but a mode laid back tuned one to become more enjoyable. In short words, cables and amplifier pairing made a big difference for HP2 and in my case were crucial to be able to properly enjoy them.
About burn in:
As stated in the previous section, when I was forced, because of the lack of XLR headphone connector on TT2, to return to the stock cable after around 20 hours of listening I felt I went back in time to the problems I had with the headphones in the beginning. So, in my view, burn in relates to the cable and not the headphone drivers.
About build quality:
The overal quality is good and the accessories provided are welcomed. However, I consider that the plastic used on the driver enclosures feels cheap and thin. The cable is also very thin and while being light, lacks any premium feel. I think the major design decision here was weight reduction so the made every compromise possible to earn some grams. The carrying case is nice and feels like good quality but I am unsure how many are taking these with them in their travel since they are not so easy to drive. Maybe sound engineers moving from one concert hall to another. I have no idea and it is not my usage scenario. The headband and suspension mechanism is also good, they are sitting firmly on your head and you can work with them without problems. I have no idea how reliable they will be, but the 5 years warranty and 100 days return period are appreciated.
What I do like and do not like:
Here I cannot make a statement without comparing with something else and my only choice is again the D8000 Pro. This morning I returned to it for only a few hours just to see what I will miss from each one of them. During these 4 days I have listened 90% to the same album, so my experience is biased by a corner case, but it is also a good point in the first days because it helps me to make some 1:1. My memory is not a very thrustworthy partner so if I start to listen to mixed compositions I usually loose the ability to compare. Anyway, what everyone expects to hear is that the bass is indeed one of major drawbacks of the HP2. I would say the mid-bass is there but in reduced quantity. The problem is with the sub-bass. However, with the 2 adjustments I have made (cable and tube amplifier) I can say that I am pleased, especially for rock or orchestral. For sure, for electronic music it might not be enough in any case. Again personal preferences. Where HP2 shines is the midrange, where everything is very detailed but also distributed somehow wide, having air between instruments and being able to render with more clarity more complex passages. Voices are somehow in front (here I might be biased by D8000 Pro which has in my view a problem with male vocals and puts them somehow behind their normal place in the recording) and acoustical instruments are very clean and sharp. But the problems come again on higher frequencies where I feel they lack natural presentation. Something is missing there and I still try to understand what is exactly the problem. I like to call them delicate and an idea metallic but for sure, it's not the proper description.
But what I like on the HP2? The transparency and ability to dissect the midrange and present it wider than other headphones. For rock music and electric guitars this is a major improvement compared to the majority of V-shaped more musical headphones (also the case of D8000 Pro). What I miss on the HP2? As I said several times, I would have liked to have more bass extension and more natural highs. However, I know that every fix in a range of frequencies will damage another range.
Conclusion:
HP2 will not be able to replace D8000 Pro. Because both are doing different things. However, while listening to D8000 pro after a pause, I was missing the crystal clear midrange of HP2. It's like HP2 deconstructs the midrange and puts it in front of you in pieces while D8000 pro let's you see it from all angles and you need to decypher each piece by yourself. While listening to HP2 I sometimes miss the more emotional and warm signature of D8000 Pro. While it was not able to replace a headphone I like, it taught me a very valid different point of view and now, because I know it, I feel something is missing on the other one. HP2 is not a good allrounder for me, but it does better on some very specific things which are very important in my case.
I do hope this made sense for everyone.