Do understand your comments, at least I think so. However, my experience is that the waves including lowest audibles can be felt and /or heard coming from a direction. Asking the question after the event, where did the event (sound) come from and one's answer must be their best estimate and I don't know being disallowed as an answer.
The first subs I auditoned were way back in time and in Seattle at a Magnolia audio store. I went back five or so times as I am a hard sell by most standards. The designs were downfiring as many were, and some still are. Nearly impossible to detect, immediately, the direction from which these low HZ originated. The sub fregencies just were in the room. The richness of sound was very much UP a notch or two. The side-firing designs, which are the two I have (Genesis Servo's), do emit sound to my ears from a direction either directly from the drivers and reflectively off objects, as I roam continuously about the listening room. With only one sub operating, what I am hearing is originating from a general area, and only one area. The walls and floor are very significant in this room and any room, we all agree? When both subs are connected and placed one on the right and one on the left (I prefer maximum separation and have settled at fourteen feet between), the low HZ are coming from a larger area. And the room is filled with more low HZ sound. And the direction that the sound originates from becomes very difficult to ascertain. Move away from SPOT and the point of origin becomes more ascertainable, including pinpointing some reflective sound and surfaces.
I decided on side-firing for one reason. I simply wanted as much of the music (sub sound) to arrive at my ears before bouncing off of other objects, that being the floor. It would seem logical that the shorted distance and most unobstructive path to the ears, when at the SPOT, produces the least delay and a positive result. Two side-firing, and set up to face the SPOT, are the best I have ever listened to, to this day.