They aren't inefficient though, unless your point of reference is a Beats Pro or A.00 or something equally insane (you know, 120-130 dB/V - like a boss). Seriously, by pre-iPood standards they really don't stand that exotically apart, and if the original reviewers back in 2006 had tried them from home hi-fi equipment or similar (which we all "know" is evil and not designed properly for use with headphones (Because if it was, companies like Corda and Woo couldn't hawk multi-thousand dollar components to us!)) instead of their mobile devices, the entire mythos about their "hard to drive" qualities would've never started up.
Of course, the converse problem is now that the HE-6 is out, and just as bad as the K1000 from years gone by, the 70x gets grouped in the same schema of "hard to drive" with it. So now you've got people convinced that you need a 6W headphone amplifier (which you don't need for anything), for 70x.