the vertical stack looks cool.
if you end up going with the vertical stack, the distances from the drivers to your ears will be a little off. if the differential distance starts to get more than 1/4 wavelength, the soundwaves won't be summing properly, at least in some sense.
this problem can be remedied in at least two ways. the first would be to physically align the drivers on an imaginary curved circle such that all the driver will be *physically* the same distance from your ear. just move the outermost woofers forward. the second way is to use delay on the in woofers and the coax in order to push them back "electronically". by adding delay to the units in the center the sound from all the sources will arrive at the same time.
such a setup would put all the drivers in perfect phase and so no shading of the outter woofers would be required.
out of align:
aligned physically, electronically, or both:
does it make a difference? will you even be able to tell? who knows? :-)~