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To my ear, HE400i is quite problematic for some type of music, especially those poorly recorded pops. Because it has a very sharp peak around 7kHz. And bare in mind that this measurement is for the old design HE400i. Moreover, the new updated design HE400i sounds even brighter than the old one. Which is to say, the peak around 7kHz is even sharper on the current version HE400i...
So why that peak you may wonder? Well, peak around 7kHz makes a headphone sounds more airy, and tremble becomes sweeter and more impressive for short daily listening session. Which is to say, you are more likely to be impressed right after you listening to HE400i, and you will like listening to it for maybe around 30mins to 1hr. But after that... It does cause more listening fatigue much quicker. So for longer listening session (>2hrs), HE400i can sound too fatiguing.
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On the other hand, HD650 sounds too boring and not exciting on some unoptimized low end to mid end system --- especially for short listening session. However, HD650 does sound great on high end system. Even for average unoptimized low/mid system, HD650 will sound smooth (not fatiguing at all) after even 3-5 hrs of listening.
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Personally, I would recommend HD600 as a better headphone than HD650 or HE400i, if you want to get both smooth and exciting sound. To my ear, HD600 is easier to be pushed than HD650 (which means, you don't need to spend as much on a HD600 on your system).
HD600's flatness of curve matched well with my own listening experience: It's neither boring, nor fatiguing. It's both engaging and easy listening.
HD600
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I have done critical listening on all the four headphones HE400i, HE400s, HD650, HD600...
To my ear,
1) HE400s sounds generally best on portable or average low / mid end system.
2) HD600 sounds best on well set low / mid end system.
3) HD650 sounds equally well as HD600 on high end system. It's becoming a personal taste preference between 650 and 600: HD650 sounds warmer and HD600 sounds less warmer --- But HD650 is not boring at all on high end system.
4) HE400i sounds best for some specific type of music, including female vocal and violins, etc..., on low end system. But it cannot be pushed as well as HE400s on portable devices (maybe because HE400i has larger impedance than HE400s).
5) HD650/HD600 doesn't sound as well as HE400s/400i on portable devices or unoptimized low/mid end system. But they do sound much better on well set low / mid end system, or a high end system.