Quite a few subs have shallow slope filters, what you are hearing could very well be normail for that design. An option would be to add another pole of crossover before the sub;. Adding a pole would sharpen the cut off, which may or may not give you the acoustic summation you are seeking. Alternately DSP the subwoofer, on the cheap a unit from Parts Express, the Dayton (tm DSP, for roughly $75