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?
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.
BTW, the one thing I don't see evidence for is the need of bass traps.

I know the world will be shocked, but this is not the case here.  What you need is the slope from 1kHz to 10kHz to go further down.  The problem is not, as often the case, that you have too many room modes.  The problem is the relative output from 100 Hz to 1 kHz is far too low relative to the rest, and that is making your system feel like there's way too little bass.

Fix that and you may not even want a sub. :)  As I wrote in my blog:

But wait, you are trying to fix a bass problem right? Well, what if the problem is not too little bass, but too much midrange or treble? A live room will sound much brighter. Reduce that and, like magic, the bass appears like a Spanish galleon emerging from the ocean at low tide.

I had a similar issue.. Solved.. Do you have 3 way tower speakers ? if so, try this.. I first found the perfect toe in.. Then I placed 3/4 in rubber pads underneath the front of my speakers to tilt it back some..  Improved my bass a lot.. Sounds crazy, maybe ? It works for me..  Note that my bass driver sits towards the bottom of the tower.. Good luck.. hope you find a solution
Good thing you are measuring your room/system. Terrible curves unfortunately. I would skip the subs or cross them at 30-40 Hz and use the Roon digital equalisation. Preferably using convolution filters. You seem to have a "loaded" room so I don't think acoustical treatment will give you much help.