I agree that the he400i isn't as efficient as some people would suggest.
I was expecting them to not be power hungry at all but was surprised that I have to turn my Asgard 2 up quite high in order to listen at a level I like (Usually about 3/4 or a bit less of the way to max). That's on low gain of course, high gain puts the volume into the stratosphere
.
Still, I was expecting the 400i to be much more efficient, I definitely can't listen out of a mobile device at a volume level I prefer. If you listen at lower volumes it's possible you might disagree with my assessment though.