Lots of bass at walls, lack of bass in center of room/listening position


I guess this is relatively common in listening system. Is there any way to smooth this out so I get more bass energy at my listening position? This happens with our without my 2x 18 inch subs. Room is 12 x 16 x 8 ft, speakers 4.5 ft apart on long axis and I am sitting 4.5 feet away. I tried moving back and forward but the entire middle center of the room except near the walls has decreased bass.
Is this a boundary effect or could it be due to bass cancellation effects?
smodtactical
@erik_squires Its in the picture above called Measurefix. The purple line is with no subs (legend on the top left). I also tried doing 1 or the other sub as well.
Here I did a curve with just mains, no subs and bit more smoothing at 1/12.
http://www.diabolicaldesign.net/nosubs.jpg
Here is another comparison, this time with left sub, 2 subs and no subs and a bit more smoothing (1/12) to make this more clear.
www.diabolicaldesign.net/smooth.jpg
What about running a DSP module just on the subs?  A friend did this with great success.
Look closely at phase between mains and subs. If you don’t have the phase right, it cuts out a hole where they should overlap due to cancellation. This same issue plagued me for a while, and while phase may not be the total solution, it’s a strong cause of “where’s the bass?” I found the lowest bass was dependent on the placement more, while the frequencies near the x-over point were very sensitive to phase.