Regarding ANR in an HGU 55 - rather than ANR I'd recommend the passive CEP system - essentially a set of earplug speakers which work in parallel with the speakers in the helmet. One of my Pitts co-owners has ANR in his HGU55, but has had much better results with the CEP earplugs, and I use them as well. Have also fitted CEP kits to DC headsets for a couple of the local Extra drivers.
http://www.cep-usa.com
Also found it easier/cheaper buying direct from CEP, but FlightSuits or Headset Services can also supply and fit the kit.
Edited: also agree that the Oregon Aero earcups & liners are better than the stock Gentex items.