Legairre, the most logical reason for this effect you hear is simply b/c they are MORE speakers in the room.
You've added subs so yes, the LFE will sound more prominent. I've had 2 subs play in the front and also hooked one up to the rear, but in keeping with a single subwoofer config, there is no change in sound. Adding the second sub did add more "prominence" to certain movie scenes. Of that observation i agree, but again for an obvious reason.
As for directionality, well, that's how the surround sounds work to trick ones ear as to LFE location. You need a range of sound, even for explosions, that go well above the 80Hz point.
If you get the chance to hook up nothing but subs in your room, unhook everything else and then try to ascertain their positions during a low 40Hz note (this has to be an equivalent note to all channels though, not just playback of a movie with some encoded material in this range as the SPL may vary for each individual channel). Unless something is rattling close to a wall, it is unlikely your ears will be able to triangulate the noise. The wavelengths are simply too long.
