Then there's the question of sure, they can reproduce 90 db at 20Hz (1 watt or so) but what happens when a reference level or higher (say 100 watt) signal gets sent out - can they really reproduce a 110 db note at 20Hz? Pretty unlikely.

As for stacking subs, lots of folks like it, but I don't. My personal best results have been (1) corners of a symetrical room and (2) a horizontal line array. Maybe this is what Kevin means by 'along a wall'?

YMMV.

Charlie