I'll need some teenage assistance to raise the boxes. With drivers they feel like 100#
Here is a graph of three close measurements.
Blue is mic 10" from the driver cone, off center halfway between phase plug and surround.
Green is mic 1/4" from surround, mic parallel to driver axis, front baffle thickness next to mic tip.
Red is mic 1/4" from driver cone halfway between phase plug and surround.
All raw measurements, no gating at all. Signal after XO filter, horn playing, but mic is outside the directivity of the horn.
Looks to me like this dip is not room acoustic phenomena.
Stuffing the bird next