Here's a fun read on line arrays: http://www.meyersound.com/support/paper ... _array.pdf
I never heard the Wall but I did hear the rig at Oxford Plains in 1988 where there was a stack of Meyer subs one wide and 4 stories tall on either side of the stage (near as I can remember and judging by comparisons to trucks parked right behind them in photos). I have to say that it was the best low end response I've heard out of any system anywhere. It was deep (sounding like it was coming up out of the ground) and quick, with no muddiness. The impact was felt all the way to the back of the racetrack.
I did have a conversation with Dave Rat, sound engineer for the Red Hot Chili Peppers, etc., when he posted some stuff on the net about difference bass cabinet configurations and he dismissed low end line arrays as inefficient and sonically inferior to his cardioid sub arrangements. I've not seen the Peppers live since the Uplift Mojo Party Plan tour, but I can't imagine that it would sound any better than what I heard at Oxford Plains. Perhaps more pummeling, but not as quick, clear and deep.
I have to say that, in general, line arrays as commonly deployed strike me as being the live sound equivalent of mp3s. There might be a lot of reasons to use them, but they have more to do with convenience than sound quality. There are exceptions, of course, but hearing most of them does make me miss old school PAs sometimes.