Lots of bass at walls, lack of bass in center of room/listening position

Diffusers work based on wavelength. That's why you see those panels with wood blocks of different sizes at different heights. Each frequency has a wavelength. The higher the frequency the shorter the wavelength. Little blocks of wood can diffuse high treble because the wavelengths are on the order of inches. The low bass you're concerned with is at a wavelength on the order of some tens of feet. Ten to forty or more feet. In order to work your diffuser needs to be at least that big. This is why we have the low bass problem in the first place: even an entire room is small compared to low bass waves!

This is also why bass traps are no solution. They can do something, just not much. The only real solution is a room that is multiples of the lowest bass frequency. A room a hundred or more feet on a side. Think about the phenomenally good bass you have experienced at a rock concert. Because: big room! The next best solution is lots of subs. Each sub still has the same mode problem. But with so many each one puts out less bass, so its mode is small, they are in different places, and together they average out to exceptionally smooth powerful bass.

Miller so in a smaller room you cannot get good bass without subs ?

...so in a smaller room you cannot get good bass without subs ?

Yes you can but you have to work harder at achieving the goal. Your room width is the same as mine and about 3 feet less in length than mine so I know it can be done. To start, get at least one if not both subs out of the corner. A couple folks posted links to videos that show you how to place subs using the crawl method. Try it out, you might be surprised. I use the method referenced by Atmasphere, but you can accomplish this with 2 subs as well, and they don't have to be big subs.

Most speakers don't provide that 'room lock' bass in my room that I hear in shop demos. I can't explain why but I had a pair of speakers on stands I couldn't get any bass from so due to the insanity of this hobby I flipped the stands over, mounted them to the ceiling than mounted the speakers to that and GOT the best bass ever! Why when the speakers were the same distance off the floor as they were off the ceiling and the same distance from the corners did it make such a huge difference? IDK, but it was embarrassing when visitors would bump their heads on them walking by. lol.
Often the best sounding systems by dealers have the speakers in the most unconventional places and if you don't have the freedom to place them where they sound best you're stuck with bass traps and digital eq along with your sub(s).  Nice speakers, seriously wanted to give them a try.

Thanks guys. Interesting anecdote about the speaker flip thing, heh.
Anyway I did some measurements. Both with subs, no subs and at my listening position and at a position that is 3 feet back and 3 feet from the left wall.
www.diabolicaldesign.net/Measure.jpg
Please have a look