I know there is a lot on here about headsets and which is the best but having flown a few GA types people seem to be getting a bit carried away over the requirement for buying the most expensive headset out there. If you are flying a standard light single (cessna or piper) for up to say 50 hours a year, then why spend £600+ on a headset? If you are a commercial pilot flying something noisy then fair enough but how is it that Bose get away with charging so much and not actually advertising their headsets NRR? If you are choosing a headset that has little passive attenuation with the electrics turned off then you are asking for trouble IMHO. The sennheiser is fine when the batteries go. I use a sennheiser in a 300 hp cessna with no problems when the batteries go flat, I know the bose has a long life (assuming the aircraft is not too noisy) but they always go flat at the most inconvenient time.
For what it is worth I know someone with over 8000 hours on piston Islanders who until not long ago had peltor 8006's at best and still passes his audiogram without problem.