The 12" downfiring sub in Def4 is cleaner, quicker, and it loads the room more evenly. It is in all ways better than the 4x10" linear sub array on earlier Definitions. Plus the internal amp is better, cleaner, more powerful -- and you have far more control over deep bass/room interations with the parametric, phase and hinge frequency variables. The rest is in the Zu Definition 3 thread here.
Phil
Phil