I have two 13.4s, one for me and one for my wife. Both have been modified with Oregon Aero's (passive) upgrade kits, and I've been very pleased with their performance.
If you install one of those kits - which require you to insert their foam product into the earcups - you will be very surprised at the el cheapo foam that DC puts into their headsets. Essentially, there's nothing between your ears and the outside noise except the plastic shell of the earcups.
AOTW: here is the link you need:
www.davidclark.com/HeadsetPgs/plugadapters.html