is there a difference between them regarding vocals? Are they similar? Is there any other difference between them in the mid range?
Every instrument has a whole lot of information in both the lower and upper parts of the midrange. This chart will help you.
The area the HE-400 is recessed in-- the upper mids-- deals mostly with the harmonics of instruments.
Thanks for the link, that was really interesting. The part where too much high mid range can cause fatigue, my ears hurt and I get headache after 10-15 minutes when listening to the HE-400 really strange how offensive they sound.
So this means the HD 600 will have more natural vocals and HE-400 will have a little more treble in vocals and a little bit artificial?
As a measured graphic, the headroom dummy has this on record:
Notice by comparison the HE400 has a ~12-15 db drop in output from 1500~6000Hz. That doesn't really mean much since each of us hear and perceive things differently. To get a feel for what this upper midrange recession sounds like, spend some time with the iTunes or winamp EQ and cut or boost bands from 1500~6000. Toggle on/off/on the EQ to compare with and without. Thats a huge chunk of the audible spectrum, and I think its partially why the HD400 have such a unique signature.
Okay, but I don't know why the HE-400 gives me headache after 10-15 minutes, I have the DT 990 pro's and they also have a lot of treble and bass, but they don't give me headache at all. It's very strange. It's not the sibilance, it's something else.
Frequency response is only a small part of the equation. I can't state it enough that no two people hear things the same, and as a result we all have our own unique sensitivities. So just because most have a sensitivity to such and such frequency, does not necessarily mean someone else will. There are other factors at work too like time domain transient responses as well even-odd order harmonic resonances. The HE-400 weighs a ton, so we can't rule out ergonomics too. The HE400 has pretty much ruler flat bass-sub bass extension, maybe its the deep bass extension you are sensitive to?
We also can't rule out listening fatigue from higher than tolerable volume levels. I used to get tinnitus from my HD580 (black baffle cloth era from pre ~2007). My RS1 never did this, despite hours of listening at a time. It turns out I was on average listening to the 580 at a louder than tolerable level, in a subliminal effort I think to make them more vibrant and exciting. So now I make sure to keep volumes in check.
The HE-400 have the type of sound-signature that wants to make you turn the volume up, because our most sensitive range (2-3khz) is the part that's recessed on the HE-400. If you turn the volume up on the HE-400, that will also amplify its treble spike, making it more of a problem than the DT990's treble spike. That's my guess.
The HE-400 is perhaps the best high spl headphone I've ever listened to, but you need to EQ the treble down for that to work right.
If you took that headroom graphic of the two headphone's FR, and elevated the HE-400 fr by about 7db to get it properly aligned with the HD600's, you'll get a better picture of how the two compare.
Another thing I'm noticing with the vocals with the HE-400 is that they sound like they have too much treble in them, almost artificial sounding, or am I crazy? Is there a frequency that is the cause of this?
No youre not crazy... see the Headroom measured treble spike to the far right at 9khz? Thats the culprit right there. That combined with the reduced 1.5-6khz midrange, while boosting 8-9khz treble can result in exaggerated "SSSSsssssSSSS" in vocals.