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
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. 
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.


Great pic.  Um, you definitely have complications.  There's an increasing trend from 60 Hz to 1 kHz.  This should be flat or descending.  Also, notice the regular ripple starting at around 6 kHz.  That indicates regular reflections in the mid-treble.

Essentially you have way too much mid bass to mid energy. If this is a highly reflective room that would explain it. Before adding the sub, fix up your mains and room.