I have been listening to HE400i for the last few hours now. Mine is the Version 2, as I bought it direct from Hifiman. I will start off by saying that this is an interesting headphone. First off, the presentation when unboxing is impressive. You would think by the box alone that this headphone costs more than it does. I have no issues with the build quality of these headphones. They are gorgeous looking, and no blemishes are present. Everything looks like it will maintain integrity over substantial hours of use. Comfort here is impressive, just as it is on the DT880. The clamping force is higher, however, the clamp literally disappears within a couple minutes, After this, I then barely notice the headphones. I have no issues with my glasses. I do have to mention though that I absolutely hate the included cable with the right angled connector. Obviously, Hifiman felt that since this headphone is 35 ohm, that many people would be willing to run it straight from portable devices. However, in testing, it is clear that this headphone need a good source, and a good amplifier to shine. In all honesty, this is clearly not a portable headphone in any regard, so a right angled cable makes absolutely no sense. I will be replacing this in the very near future.
In terms of sound quality, it is quite different from the DT880 in some ways, and similar in others. The HE400i is a better sounding headphone, no question. The bass on the HE400i is capable of better extension, and does not have the same level of sub bass roll off when compared to the DT880. At this point, without many hours of burn in, I would describe the HE400i as a "groovilizer". The bass is not insane with authority, but it is tight, fast, accurate, and punchy. If the music asks for it, it easily delivers. It just does not attempt to overpower the other frequencies with its presence. This is the level and punch of bass that I like. I have been tapping my feet to music for the last 3 hours. It is really a grooving headphone. The bass does not bleed into the mids even the slightest. Now, oh my, the midrange. I have seen statements claiming that the HE400i is recessed in the mids. There is no bloody way this is true. These people are clearly using trash onboard audio, or another trash source/amp. The mids on the HE400i are lush, rich, and musical.
I have always enjoyed the midrange on the DT880, but this is just a straight up improvement across the board. What the HE400i offers, is all male and female vocals, are much more up front. There is a lushness and beauty to the presentation. Everything is so cohesive and transparent. It reminds me of sitting in a smaller concert hall, and being a couple rows directly away from the vocalist. I tested both headphones several times to hear the differences. Imaging is also superior on the HE400i. Instruments are nicely separated, and location is very realistic. In terms of soundstage depth, the HE400i is the winner. In terms of soundstage width, the DT880 is a bit wider. The HE400i is not quite as wide, but it is more focused, with with ability to reveal smaller details. Now, regarding the treble, these two headphones both share an 8kHz spike. However, I find the spike on the HE400i to be more sibilant. The DT880 also has more detail in the upper treble at 14kHz-16kHz. The HE400i is rolled off somewhat in that region. The HE400i is certainly a warmer sounding headphone, but I would classify both headphones as neutral. One other thing to note, is that I am surprised at the power needs of the HE400i. On my M-Stage HPA-2, both the DT880 250 Ohm Premium and the HE400i actually require the same amount on the dial. Literally, it is almost identical. I expected the Hifiman to require somewhat less being 35 ohm. She definitely likes to be well driven.
In terms of the treble, I am using the following EQ. Edit: I have updated the EQ again.
Edit: Be aware, that after using this EQ adjustment, the bass will also end up becoming more impactful with improved extension as a result.
7kHz: - 2
8kHz: - 2
10kHz: + 1
16kHz: + 2
Other than this, no other EQ change is needed. I just cant deny the sound quality this headphone offers. If anyone is noticing what I am with the treble, just give this a try with an EQ. It just evens the treble right out.
Edit: In case anyone is wondering, this is my setup.
DAC: Creative ZXR with all 4 opamps upgraded to the LME49710 Metal Can opamps on adapters.
Headphone Amp: M-Stage HPA-2 with opamp upgrade to LME49990 on adapter