This is the spec for the HXL which looks similar to the photographs of the unit I picked up:
http://www.davidclark.com/avHeadsetS...Name=H10-56HXL
It says M7-A amplified electret microphone, which is the same used across their ENC range, including the one with the GA plugs.
The link you supplied above says amplified dynamic microphone. So you can see why I'm getting a little confused when it comes to impedances. I'm going to pick my headset up tonight so hopefully I'll be able to tell what's going on a little better.