I've noticed there's a lot of confusion between things like impedance and sensitivity, or current and voltage.  To compound matters, whenever someone asks "will X headphone amp drive Y", there is traditionally a lot of misinformation and muddlement that goes on that eventually causes the OP to leave audiophilia for a more pleasant hobby, such as recreational self-waterboarding. 


Here's an article written by Pete Millett that clears this up in a pretty simple manner.  For those averse to doing the math, there's even a handy downloadable spreadsheet at the end.


http://www.apexhifi.com/specs.html


Please note that this does not mean that all watts are created equal and that the biggest compensators monoblocks possible are automatically the best solution for your inefficient headphones.