How can a subwoofer expand the soundstage?

I have recently added a Rel Storm III to my two channel system (Essence Sapphire amp, modded Essence Reference Super Gems, Arcam FMJ 23T CD, SOTA Sapphire TT, Benz Glider, Audible Illusions Preamp). I immediately noticed an improvement in imaging and depth of the sound stage at all frequencies. The REL manual specifically mentions these benefits. Can someone please explain how adding a driver that's designed to play at lower frequencies can have this kind of impact? I will NEVER take the REL out of my system.