Voltage sensitivity is taken at some reference frequency, typically 1khz. The frequency response chart shows you the output of the headphone though a range frequencies given a constant voltage. The current draw can be concluded from the impedance graph of the headphone. The O2 should be able to power those headphones to near defeaning levels. Interestingly enough, the HE-6 as measured by Tyll is far less sensitive, with an output of 77dB/mW and a 90dB output at 1 volt.
5/9/14 at 9:57am