I had a DC 13.4 standard passive headset and recently bought the add-on ANR kit from Adams Aviation. It cost around £140 and required some nifty work with a soldering iron. The ANR is absolutely brilliant, but I think that when the ANR is off the passive attenuation is less good than it was before. It might be to do with having to remove some of the insulation material to fit the slightly bulkier ANR electronics into the ear cup.
Much recommend the ANR bit, though!