Chickenhawk,
I had my H10-13XL converted to H10-66XL's, in effect this means that they are now switchable between high and low impedence.
The entire mike boom needs to be changed and they add a small toggle switch to the outside of one of the earpieces as well as other stuff inside the headset.
I fly a L1 Super Puma and they work v well, good passive reduction when the batteries fail, usually last about 2 normal days. Also v Good ENC when you vacate the aircrafts for Rotor Run turn around.
Only downside is that the work was done by Headset Services Ltd in the UK and the turnaround was v slow, 10 weeks. Also the price was on the high side £201, but they did a very professional job of the conversion. So I guess you get what you pay for.
I tried the Standard issue Peltor's with earplugs, but I found the radio too quite even on max volume. The DC's converted allow much better control of internal comms and radio, which would enable me to wear the earplugs and hear the comms OK with the DC's.
Hope this helps