I've heard a couple of different ML speakers, but never the Preface line. The ones I spent the most time with, when comparison shopping against M80's, were the Purity
. This was at the local Best Buy store, in their Magnolia 'showroom'. I really wasn't all that impressed.
While I do like the aesthetics of the speakers (they look cool), I felt that the overall sound was lacking. My biggest complaint was that they had *no* bass. Basically had about as much bass as a pair of earbud headphones from across the room. Way less bass than even what M22's can produce. I fiddled around with settings on several different receivers, including a model similar to the one I own at home, but couldn't improve it at all. Other speakers had good bass, so I could only conclude that they just don't produce much, were incorrectly hooked up, or that maybe there was something actually wrong with them. The salesperson said that's how they were supposed to sound, so who knows.
The other thing I didn't like is how extremely directional they were. Yes, as you said, that sweet spot sounds good. Nice and wide soundstage and the speakers disappear nicely into the room. But from any other spot in the room, they just didn't sound special to me at all. Very 'blah'. With my Axiom's, my entire couch is the sweet spot, and they sound great from anywhere in my house.
My $.02 anyway, take it for what it's worth.