Interesting this thread coming up....I'm doing some research on helmets for an article. Seems it's the low frequency noise that eventually leads to the loss of hearing in the high frequency band.
Gentex advises the dBA protection for each of its helmets. Helmet Integrated Systems advise a general dBA reduction and will advise the protection for each frequency if you ask. Gallet just didn't reply to my email. ANR is good but it seems CEP is just as good if not better and much cheaper. I had trouble getting my CEP to be comfortable but with some changing of the ear plug sizes, I now find them quite good.
The HGU56 meets the latest US Army spec.
My advice is to get the latest data from each of the manufacturers and compare them ... weight, dBA protection and impact reistance. Suggest CEP or ANR as well.