There is no reason to use a phase plug in front of a woofer. Its job is to increase high frequency extension, not something you want in a basshorn. Even midbass would be something not worthwhile. Midrange, possibly. Tweeter, definitely.

The whole purpose of a phase plug is to make path length the same from all points on the cone/diaphragm to the throat. Without one, sound eminating from the edge travels futher than sound coming from the center. The difference causes a phase shift and if enough, cancellation. That's why high frequency suffers without a phase plug.

Rgs, JLH