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?
@erik_squires: I only have a delay knob on my subs (PSA V1800). Is that what I should play with? I don't have polarity or phase controls.
@millercarbon Oh wow different subs? I thought I wouldn't be able to do that. Which subs do you have. Do you find they give you clean bass with your speakers and are fast enough to keep up?

Here are measurements during each sub off and then both on from the listening position:
I wonder if diffusers behind my speakers would help? In addition to bass traps. Because I can't pull my speakers out further than a foot off the front wall.

Lets say the diffusers, and traps and everything killed all the reflective problems, you have, what do you have left? 

Only what's coming from the baffle face, even the back side of a dipole would be killed.

So taking what MC said the wave lengths and killing the rest, you can only do a few things. Make a little bass everywhere, very even, very controlled, but everywhere. That's out, can't clutter the floor.

or You could EQ the bass, why? Your killing the reflective excess, so you can increase the actual bass in the seated position. BUT you can't kill all the reflective surfaces, so that's kinds out.

But the bass comes after the mids, and the tweets before the mids, so what do you do? You HAVE to move the speakers. NOT I can only move them this much, there is no other option. If you want to fix the problem..

The bass needs to be closer to you, the mids and highs need to be further away. TIMING.... they (lows mids and highs) need to arrive at your ears, at the same time.

Delay and phase in reference to a subwoofer are the same thing. Delay in this instance is the more accurate description of what the control is performing.

You have an unconventional listening space which may require an unconventional solution. Try what ever you can think of and see what happens. Aim your subs in different directions, Raise them off the floor, Raise them off the floor at different heights to each other, change the low pass filter setting (crossover point). There could be some cancelation caused by the interaction with the woofer in your mains. Try asymmetrical crossover points. If your speakers have ports try plugging and unplugging in various combinations. I'm sure there are other things you can try all you can do is see what happens.

In my system in a 14 x 10 x 9 room I have one sub sitting right next to the listening position aimed towards the main speakers and another sub almost directly behind that sub and aimed 90 degrees from it.
In general, your measurements show the two are not integrated. You want the bottom and upper part of this graph to merge completely seamlessly.

I only have a delay knob on my subs (PSA V1800). Is that what I should play with? I don't have polarity or phase controls.

My suggestion:

  1. Minimize the phase setting.
  2. Flip the wiring to the main speakers to compare.
  3. Pick whichever wiring gives better response.
  4. Adjust the phase dial from that point to eliminate null.
  5. Adjust sub level to elimiate step between sub and mains.

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.