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?
@millercarbon thanks man, I will play with it.
Oh and I forgot to measure the front wall to the front baffle (I was doing to the back of the speaker). My speakers are 28 inches away from the front wall.
I am gonna try to play with the toe, lateral movement and the forward and back movement until I get the low end fuller while retaining the rest of the range. And once I hear something good I'll measure and compare. I think you guys are right in saying placement is the key here.
@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.
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.
What about running a DSP module just on the subs?  A friend did this with great success.