efficient coaxial speakers?

jeh1:  if you're going for very high efficiency because you'll be powering them with the head unit, make sure you listen to the speakers first under those low power conditions.  Normal listening tests are done using amplifiers and the results may be different.Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

   I plan to use these highly efficient speakers to be driven with an amplifier that I have built.. not too many watts, maybe 12 watts(RMS) x 2. Do you have any information on how they measure sensitivity values of these kinds of speakers? are you saying that there may be some curve to the sensitivy range depending on power input? thanks for all of your good input.