Its probably actually the recordings themselves, the 400i's will pick up more detail then the HD600 including the distortion you never noticed before.
Not really. I've owned he400i before and owning hd600 currently.
To my ear, under an average low-fi system, he400i has better airy tremble sweetness, better instruments separation, a faster speed. But hd600 has a more easy listening sound (less fatigue), also picking up more details (because he400i has some very apparent downslope in its FR, yet hd600 shows a very neutral accurate sound).
So I think it's highly likely that, you are treating the artificially boosted tremble peak around 7khz as neutral, which results you to think the flat neutral hd600 sounds unnatural or has less detail. Additionally, it's likely that you've been listening to music not having that much details above 9khz, which results you not being able to compare he400i with hd600 at that range of frequency.
To better show you what I'm saying, here are the measurements:
he400i
hd600
As you can see, he400i does enhance some frequency a lot, but also weakens other frequency a lot. Those being enhanced are what made you think it's showing you more details, yet those weaken parts are those you are not being able to hear or notice before. On the other hand, hd600 does not enhance anything.
I think both headphones are good under low-fi system, just hd600 becomes great once the system is above low-fi level. But you really have to hear it yourself, before understanding or agreeing with what I'm saying.
The sound from a truly well set system on hd600 is not just gold --- it's diamond.
I'm not saying your system is low-fi for other headphone, just that based on your impression, I think your system might not work well with hd600. Which is to say, your impression matched with mine on a low-fi system on hd600.
So I would recommend you trying a better set (or even some much better set to give it a better shot), before judging hd600 so early.
