The greater the stereo separation, the more exaggerated the left-right panning, the more pronounced the effect of crossfeed will be.
Try some old stereo Beatles with all-the-way-left and all-the-way-right images in the mix, and the effect of switching crossfeed on is dramatic. A good example track that I usually use crossfeed with (if I have crossfeed available) is "Change" by Blind Melon--play it with headphones on, and you'll immediately know why. (By the way, without crossfeed on, it's fun to shift balance all the way left and listen to Shannon Hoon a capella for the better part of the first minute of the song.)
Thanks Jude for the explanation. Will try out some old recordings and play with the filters. Cheers.