Slightly Off Topic, but still kind of relevant. I remember when an audiologist came to the UK base I was working at to measure Helo cockpit noise levels.
Pilots were wanting company to splash out on Bose ANC headsets.
Audiologist agreed that noise levels were excessive, but said that Bose was not the way to go. They do cancel out low freq well, but evidently not high freq, and they don't have very good clamping pressure for passive attenuation.
His opinion was that a helmet was best, and a good DC headset (best clamping pressure he said) with GOOD ear plugs. Just crank up the radio volume to hear it.
I use ER-4P earphones from Etymotic Research for audio enjoyment, and they have 25dB passive attenuation. If their earphones are anything to go by, then their earplugs should be fantastic.
They might be expensive, but you only have two ears!