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
Guys thanks for all the suggestions. I will try them and keep tweaking. I did play with the delay of the subs based on what Tom wrote at PSA where he said you should add about 1msec delay for every foot away the sub is from you. So I have 1 sub that is 3 feet away facing me and 1 sub in the back left corner that is 9 feet away so I did adjust that. Can't really tell if that made a big difference though. Maybe slightly better.
But I think Erik is right that the only true way to fix this is to move my speakers around and likely get them more off the front wall. Right now they are about 10 inches off the wall and they probably need to be moved out more.
Here is updated measurements and I zoomed the axis so you can better see the contours.
@audiorusty, interesting idea about the variable crossover points on the subs. That seems cool. The acoustic fields guy also recommended subs at different heights to make things smoother.

www.diabolicaldesign.net/measurefix.jpg
I did play with the delay of the subs based on what Tom wrote at PSA where he said you should add about 1msec delay for every foot away the sub is from you. So I have 1 sub that is 3 feet away facing me and 1 sub in the back left corner that is 9 feet away so I did adjust that. Can't really tell if that made a big difference though.

Of course not. Because he's full of it. The one foot per millisecond is based on being approximately the speed of sound in air. One foot per millisecond.  

There's a couple times this matters. One is side wall reflections. Side wall reflections that arrive within about a 3 to 5 ms window of the direct sound have a very bad effect on imaging. That is why we want our speakers a good 3 feet away from walls. The other time is with movies. If the screen is several feet closer or farther away from you than the speakers then the people talking on screen will be out of synch. That's it. 

This rule of his is not applicable to subs for the simple reason human beings cannot even hear low bass at all, at less than a full wave. Since a 20Hz wave is 1/20th of a second that is 50ms before we even hear it. So when it comes to subwoofer bass you can forget about timing. Its simply not a factor.
Humor me.  Turn the sub off and send me a pic of your mains full-range.
@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.