Dipoles don't send bass off to the sides, just front to back in the room. Most rooms suffer from "head end ringing," where the bass coming off the sides of the speaker bounces up and down off the ceiling and back and forth side to side off the side walls a number of times before reaching the listener late.
A good sealed box or tuned port woofer will not drag or sound boxy in and of itself, but it will tend to light up more room reflections and likely introduce longer bass decay times in the room, which can make bass sound slow and less clear. I've not heard dipole dynamic woofer arrays but I've heard they can keep up and blend nicely with electrostatic speakers and add some clean solid bottom end extension. I've heard Magnepan is experimenting with a woofer array of some sort and Guttenberg says it sounds great; https://www.youtube.com/watch?v=AzPBz3SOT4A