Yeah, sorry Pete -- I should have qualified my statement a bit further!
That IB setup you linked to is a good one, but isn't ideal sonically. In fact, the best IB setup is simply to screw the drivers flat into a wall (albeit a reinforced one!). With the manifold setups like the Shiva, the opening should be equal to the summed area of the drivers (which that obviously isn't) and the drivers themselves should be as close to the opening as possible (or you get interference issues within the manifold). People tend to build manifolds as they're very neat and still do a fabulous job... but not 100%.
Whatever you find pleasant to listen to is right for you, and some say IB is overkill. (So says he, sitting next to 4 x 18" Soundsplinters, waiting for an in-line installation
)...