fEARful modularity

The best approach to stacking cabs that have midrange drivers is to reverse the baffle of one of them. This is often referred to as mirror-image pairing.

Then you can stack the fEARfuls so that the upper cab's midrange driver is directly above the lower cab's midrange source.

Because the same wavelength- and positional-based audio principles address high frequency sources, the fEARful waveguides also are placed for this.

Here's a 12/6cube on top of a 15/6:

Another possibility that largely respects the same audio principles is to stack a mirror-image pair on their sides.

In this case it's often better to turn down the lower waveguide if possible.

Also, fEarful "subs" are designed to be used with both 12/6 and 15/6 cabs.

In this case the upper cab can be stacked with the midrange driver at the top, or at the bottom, upside down.

Try both!