I just heard about some ear buds which are quite efficient. They use multiple drivers for each frequency ranges.
Here is a link to a description of them:
<http://news.cnet.com/8301-13645_3-20005384-47.html>
More here:
<http://www.jhaudio.com/store/>
I think it would be fun to test them in the field.
73,
Kevin. KD5ONS