Never heard the Defcon IV's, but the Addictions are amazing. They are heavy and they hang low, however. If you remember, my old tower swayed pretty bad with the older defcon III's and they were a similar weight. No stress cracks though. I think the sound would be good with either, here is the short and simple dilema:
Addictions: Heavy, hang low, easy to power (250 watts per side)Sound great
Defcon: Lighter, Don't hang to low, very power hungary, assuming they sound great.