The stock kit that CEP supply for fitting to an HGU-55 helmet will work perfectly with the standard DC ear-cup shell - you just need to make sure you drill the hole in an area with minimal curvature, so that the socket fits flush with the surface (and you have sufficient screw thread on the inside available to fit the securing nut). Actually, easier to fit to a DC headset than a helmet, in my experience.

Yes, I understand. So do I understand correctly that after fitting the CEP kit the standard ear cup speakers will still work. So I then can connect the earplugs if required without changing anything?

I looked at the fitting instructions and e-mailed CEP USA about this but got no reply.

Yes, and yes. The standard speakers continue to work perfectly if you don't connect the CEP earpieces, and you don't need to change anything to use the CEP system - just plug the earpieces into your ears (in cold weather, warm them first), don headset/helmet, plug other end into the socket on the outside of the headset/helmet. Sorted.

Thanks eharding for clearing that up. DC's with optional use of CEP's is exactly what I'm looking for.

The Dutch version of your NOTAM site is much appreciated too. :ok:

Thinking laterally. Would a helmet as used by rally drivers be suitable? Light weight, good audio quality, designed for a dynamic enviroment etc
Racewear, Helmets and intercom systems from DMRR