With two speakers playing a tone, one can easily observe cancellation at various points in the room. The location of those points and the strength of the cancellation depend on the frequency of the tone, the phase difference between the speakers, and reflections from walls. It's a very simple experiment that everybody should do as a child. If playing a more complex signal containing multiple frequency components, the cancellation points will be different for each. In an anechoic room with one speaker playing the inverse of the other, any point equidistant from both speakers will have (nearly) full cancellation. In a normal room, the amount of reflections mean the cancellation will never be particularly strong, and that's generally a good thing.To get near 100% destructive interference, you'd need to have the speakers at exactly the same distance from the listener, and the room + placement of the speakers would have to be designed to give (near) perfect symmetry of all reflections. Assuming that one of the speakers have the signal shifted 180 degrees. And then you'd only get a single "dead" spot at a strict listening position.
That's what I'd imagine. I could be wrong though. Acoustics are still a bit of a mystery to me.