After about 9 months with a Bose X in a S61, I went back to my old DC. It produced a resonant feedback if the sliding window wasn't perfectly closed, had poor battery life and just didn't seem to cope well with the low frequency noises in the S61. Other pilots agreed with me, while some others loved it, so it may just boil down to head shape and earphone seal.
It took a while to go for it, because of the above, but I've recently bought a new A20 and I'm using it in a Super Puma. So far I love it and it has given no indication of similar downsides, but it's not comparing like for like aircraft-wise.
Its the best headset I think. Its light, comfortable, can connect to cellphone, listen to music, etc... It gets quit, but you can still hear some from engines and gearbox. The headset itself, adjusts according to the noise environment. I use them in the S-61N, it performs very well, and all the pilots in the company seem to be very happy with it. A fresh set of batteries runs for about 45 hours. I also use it when flying as passenger, and it just gets very quit. My ears was very tired of noise after a long day in the S-61N, but not anymore. god luck with your choice.
museum? I guess you're flying such a hi-tech aircraft i gathered
Peltor's are good. Passive noise protection on them are better than the passive on some of newer ANR headsets. Imo, they're ok on the aircraft you're flying. DC are similar but are slightly heavy for longer flights. Bose X, active noise cancellation are good but batteries doesn't last long and it's passive are less than desirable.
If the sliding window is a littlebit open, that would make a difference. You can hear the noise from the window, but its no interference with the noise reduction system, its just the sound of the passing of the air caused by the slot. But as soon as its closed, its ok pretty quit. But still if you move the head close to the window, one can hear the air passing by on the outside. I havent tried full open yet, during flight, for some reason. only during taxi due to exessive heat in cocpit on warm days. But then there is no interference, just a difference in the noise level. The passive noise reduction is not good at all. So make sure you always have spare batteries with you.
Try a Bose QC15 in the aircraft with noise cancelling, if you like it try getting the UFLYMike attachment. I have not used this item in a helo yet but have found it to be the best set up ever. I also have the new Bose x , it works well but it is heavier and hotter.