Here is an example to ponder. My amp is 50wpc at 8 Ohms. Driving the HE500(38 Ohms) at normal listening require volume knob at 10:30 to 11(occasionally). Take a strong HP amp with rating of 1 watt at 50 Ohms, how much power do you think it's dumping into the HPs at normal listening level? I wonder this myself too but can't seem to find any solid formula yet.
Also, one designed and made the headphones, the other tests and listens to lots of headphones. Which one should we believe? Care to vote, anyone?