Polarity and phase are not equivalent. When crossing speakers over to a sub, the speakers and the sub ought to be in phase at the cross-over frequency. Otherwise amplitude is diminished and the output may be characterized as slow or sluggish.

This is what's got me concerned about Axiom's ported subs. They don't have a phase control like the sealed subs.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated