Regarding the sensitivity thing - it's all mythological. The HD 600/650 are FAR more sensitive, the DT 880 is somewhere in the middle. The K701 are half as sensitive as the RS-1 - and that's it. I think the entire source of the myth is that back in the day people had very crappy amplifiers (headphone amps by and large haven't really improved, but thanks to TI and Maxim we have some SoC stuff that is really good) or drove everything raw from their iPods. And when you have a belief system that bases loudness/sensitivity/quality/etc based on relative volume control positions (which is bunk), the K701 become this unslayable dragon.
To be completely candid, my portable (which isn't an iPod) will drive the K701 all day, and they sound no better or worse than driven from a 200wpc amplifier (tell me I was underamping them with that!).
It's just one of those nonsense myths, like Grados only being for rock, or Bose being (simultaneously (I love Bose myths because they're so incoherent and foamy)) bass boosted and bass-less.
Sensitivity or driveability?
If the former I agree, if the latter I disagree.
On that same note does anyone know how to calculate the amount of voltage/current swing any particular headphone will need for proper driveability? (I'm going to gather it involves sensitivity, impedance and a particular volume level.)