Subwoofer Frequency Response question

I just started testing my frequency response with a Radio Shack SPL meter (digital). My subwoofer is a Paradigm PW-1100 (10 in) with Green Mountain Europa speakers. The room is small, 12 X 14 X 9. The frequency response is generally very good with bass response down to 25 Hz. My issue is I have a dropout of nearly 20 db starting at 45 hz through about 70 hz. I have not been able to change it by moving the sub or changing the settings (phase and crossover point). I also have the sub on an elevated platform of 1 1/4 inch MDF with spikes between sub and platform.

I am wondering if adding internal bracing and additional acoustic dampening material to the sub woofer would change the response characteristics in a positive way.

Any ideas would be appreciated.