Aligning subwoofer bass waves so they don't actually cancel each other out is a consideration if running more than one sub.

 

For sure. It's called keeping them in phase with each other.

 

Right, which is related to distance from each other. Your Pioneer powered subs only have a button to put them in one phase, or opposite phase. So unless you have them exactly half a wavelength apart, you can't use that button to put them back in phase. So unless your hu or amp has phase shift feature, its gonna be difficult to align them. You'll have to carefully install them half wavelength apart. There may not be room for that.