I have yet to hear the Prefaces, but they are getting a fair share of airtime at another forum. Whatever you do, make sure to match at least the fronts, if not the rest.
The best match would be a 3rd Preface for your center speaker. Nothing can possibly be a better match, or implementation. Less that, the Fresco-i is the next best match (though some still opt for tweeter/stat hybrids).
I've got one for sale here:
http://forums.audioholics.com/forums/showthread.php?t=47990
looks 100% perfect with grille on except for a small blemish on the bottom. perfect working condition, and the picture makes it look about 100 times worse than in person. cheers.
feel free to PM with either questions regarding ad, or really for any other advice that you might seek. I use ML stats for the stereo.