You may be able to hear your rig perfectly, but what about the audience? I think of it like the famous double slit wave experiment:
This is with light, but same applies to sound waves, where bright=loud and dark=quiet. Imagine this diagram looking top down at the stage and crowd with two speakers side-by-side. If you have the drivers side by side, there will be spots in the room where they cancel each other out, and other spots where they combine to make it loud. One audience member might not hear the bass properly, but move a few metres to the side and the sound changes.
If you rotate that in your mind so you are now looking at a side view of the room with the speakers stacked vertically. Now the sound is much more uniform as the peaks and troughs in the combined waves only vary vertically and as long as you don't have a dead spot at typical ear level, all should sound good.
Of course it's much more complicated than this because you have many different frequencies at the same time, plus not everybody is standing at the same distance from the stage. Also the wavelengths of sound are much longer than sound - not entirely sure how that would affect it but I guess it would mean the dead spots would be much further apart. It's been a long time since I did any physics at school!