NSEU
Shouldn't Boeing and your company be advising you on which types to use? Not many aural warnings go through the headsets, so you need to know how much useful ambient sound is reaching your ears.
It could be I don't know what I'm talking about but --- don't noise cancelling headsets rely on a constant ambient noise level to set their attenuation threshold. An aural warning through a speaker wouldn't be attenuated. No?