Apparently the best form of hearing protection available is a good passive headset used in combination with tailored ear plugs. These ear plugs contain a filter that allows the voice frequencies to be attenuated less than other frequencies. They are the only device that guarentees a level of noise attenuation. On paper the clarity aloft types seem to be equivalent. The idea of not having to wear a headset in hot weather is very attractive.
Whilst the Bose seems to operate within a wide frequency range, it seems that it offers no protection for frequencies out with that band whether the battery is flat or not. I have no idea what frequency range one is exposed to in a GA cockpit, but I would be surprised if all the noise in a GA cockpit lies within the Bose frequency band.
Flying schools as employers by law must ensure that their instructors have adequate hearing protection. It would be interesting if someone has done the research into this and actually determined the adequacy of popular headsets.