A speaker is given a power rating at a certain load. It's almost always given at 4 or 8 ohms. Let's call this load rating A.
Now take your headphone impedance. We'll call that B.
Take your amp power rating and multiply by (A/B) to get your (approximate*) max power into the headphones.
For example, we have a speaker amp rated for 40W @ 8 ohms, and a headphone of 32 ohms nominal impedance. Our (A/B) ratio is 8/32 = 1/4 = 0.25
Thus our max power rating into the headphone is 40W * 0.25 = 10W
*note however that max ratings are limited by different things: voltage limit and current limit. If your max speaker load is given for both the 4 and 8 ohm load, the safer bet is to run the calculation off the 8 ohm load.
*or who gives a flippin rat's patootie, because you're not using more than the first 2W anyways. I use a little 24W desktop amp and it is more than enough.
Yeah, I think for most (sane) people you'll risk damaging your hearing before you risk damaging the drivers. That said, please turn the volume down before turning on the amp. If you're particularly paranoid about power-on transients, then unplug the headphone when you turn it on.
Perfectly explained. It's a ratio * rated power. Thanks guys.