Hi,
I have now had two pair of anr headsets, the first was a add on to my 13.4. It was very effective but found it to be not robust enough for example the connection to the battery box kept braking. So if you go for an add on make sure it is going to be strong enough.
I now have a pair of David Clark H10-13XL, to start with I had problems with the active noise canceling. DC had a design fault, they have now fixed it and they sent me a brand new headset.
I have been using it for about 4 months and haven't missed abeat.
This is a good robust unit, it also comes with a 5 year warranty.
Pacific Avionics ( Bankstown ) is the Australia DC warranty Repairer.
The only problem is that it takes 6 AA batteries that have to replaced on a semi regular basis.
Hope this helps.
Cheers