Impedance...
Folks,
Regardless of the plug, there are basically two impedances used on audio systems i aircraft. One correpsponds to using a dynamic microphone, one to an elecret type. Best thing to have in your helmet is a dynamic microphone (better sound quality, generally) and a switchable adapter. Apart from the plug diameter, this will make it work in all audio systems. The amplifier is a simple little thingy with one transistor and a few other components.
Dynamic
~ 250 ohm Impedance
Usually differential, which means that it has a natural noise cancellation for sound coming in equally on the front and the mouth end of the membrane.
Elecret
~ 5 ohm impedance
Needs power supply from intercom. Smaller in size. Sharper in sound, rough at the high end.
Also, if you experience problems with sound levels in the passengers' ears, try using exactly the same model headsets for all passengers as they are usually connected in parallel. Good old units like the KMA24-series also run the co-pilot in parallel with the passengers when the pilot isolates himself and the co-pilot ends up chatting with the passengers.
Just my two bucks worth (inflation).
/p