This might be a shot in the dark, but one of the obvious selling points of the HE-400i aside from the sound is the comfort. Does anyone have experience with the 400i and the AD900? Those are the first high-end headphones I bought, and their comfort still can't be matched by any other can, even Beyerdynamic.
I don't exactly have the AD900, but I do have the Sennheiser HD598, which many regard as extremely comfortable. I can definitely tell you that the 400i is not lacking in comfort compared to the HD598. When I demoed it and listened to it for about a half-hour straight, I didn't feel any fatigue on my head at all. I'm confident that I could wear the 400i for hours without any discomfort. And even though the 400i weighs something like 12-13 oz, it sure doesn't feel like that - the weight is distributed extremely well across your head.