Folks,
As I more or less said before, the is a very good reason why David Clark headsets are predominant in military/professional use, both fixed and rotary wing. Something over 60% of the western world market, according to the D-C blurb, and I can believe it.
If anybody is going to use a non-TSO headset on anything from Part 135 (or equivalent, or in some countries, any aeronautical radio) up, I would suggest you need specific regulatory approval.
In at least one case, of which I have detailed knowledge, Boeing amended a C.of A. to incorporate a customer choice of ANR headset ("non-TSO") as part of the aircraft equipment, thereby covering this use under the production certificate. No fault of Boeing, the (European made) headset was 'orrible in practice, made HF almost impossible -- but apparently very good for listening to the stereo at home.
One thing that seems to be emerging, ANR headsets do not provide the protection from long term hearing loss, that a really good natural attenuation headset does. As to what frequencies ANR headsets deal with, that is not even constant within the range of an individual manufacturer.
The "definition" of ANR seems to be about as settled as the definition of "organic beer".
Tootle pip!!