I googled "sennheiser hd 650 sensitivity" and got "The sensitivity of the Sennheiser HD 650 headphones is 105.10 dB/V SPL by the voltage..."
The big question is "how loud is adequate?" For loudspeakers, a fairly normal listening level is 82dB; for typical well-recorded music you'd want another 14dB headroom, so 96dB is an answer, which corresponds to 0.35 volts rms.
