During his workshop at GearFest ’11, producer/engineer David Isaac (Whitney Houston, Aretha Franklin, Gladys Knight, Eric Clapton, Marcus Miller, and many, many more) discussed how he enhances the sound of his tracks by placing them in a space, rather than having them be a small, mono source in the stereo field.
One tool he uses? A very short delay, designed to simulate the Haas Effect. Try this: pan a mono track away from center. Create a mono return channel panned the same amount off center, to the other side of the stereo field. (If the orignal is panned left, pan the aux to the right.) Bus the original track through the aux return channel. Insert a delay on the aux return channel and set it for an extremely short delay time — just a few milliseconds.
This simulates the effect of hearing the mono source in a room; blend in reverb to taste to complete the effect.