Tried a few different positions for the sub. Out of the available spots, the corner behind the seating yielded this result. Crossing my fingers for no sound localization from that area though.
It's as close as I can get to a flat response curve before doing any EQing.
I don't understand why I don't have more volume from the 15 to 30 Hz range though. The 15" Velo is plenty powerful. Anyway it is likely a simple explanation for it.