Dean, what DPLIIx and the other similar ambience extraction modes do is to remove out-of-phase sound(representing reflections from directions other than the front)from the front channels in stereo material and steer it to the appropriate surrounds. The effect varies with the amount of surround ambience that has been mixed in to the front channels(the Dimension control can adjust for this to some extent)but I've found that it improves nearly all of my classical CDs and I always use it.
One experiment that might interest you is to disconnect all speakers except the mains, but still show them on your receiver, and then apply DPLIIx to hear the effect on the mains alone of the material being extracted.
