The only relative comparison is to use the same F15 driver in an Imperial and then compare it to the ZF15L baffle.  The Imperial is going to have more bass and the bass will be just as tight as it is in the baffle.  

Steve

As a followup, since the fs of the F15 driver is around 50Hz, much of the Imperial's potential for 30Hz output will be diminished. Even so it will sound much deeper.

That said, the imperial likes large spaces. A small to medium sized listening room will generally not work because the size of the wavefront is so large it creates big standing waves that can cancel most of the lower bass. Even in a larger space, one imperial usually sounds like more bass than two due to less cancellation.

The Dipole effect of the open baffle creates two equally large wavefronts that are out of phase which cancel the base much differently in the room.  Rather than the bass having to go across the room, hit the rear wall and come back and hit the front wall and then return to cancel somewhere in the middle, the open baffle will ultimately create a larger number of smaller standing waves which is what you want compared to a couple giant ones.  Therefor in a lot of rooms, the bass from the ZF15L will probably be higher at the listening position.

Steve