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
Probably I need to keep playing with speaker positioning as you guys are saying. I'll try to keep moving them around. Right now they are 28 inches off the front wall with 10 degree toe in.
You can play with them till the end of time, no amount of moving them around will have the least effect on the underlying physics. Been there. Done that. Found a better way. Don't do that no mo.
OP:
We haven’t discussed your main speakers. What are they and where are they? It is possible they need more rear wall reinforcement. You also, 100% need mid-high frequency damping and dispersion.

If the former is true, moving the speakers towards the rear wall should help reduce the bass-mid imbalance. That is, you should get more bass out of these speakers.

Some one else mentioned Roon. Yes, if you have access to DSP and the other suggestions are not available to try first, this can help, but convolution filters are WAY overboard here. Simple parametric filters will fix everything you have, and not overtax your server.

However, room acoustics are the first place to go. You can’t fix your mid-HF hash with an EQ.

Also, the sub response honestly looks really good.  I was expecting to see a lot more garbage.  This is why I am suggesting that for now, you ignore it, and fix the mains.  That's where your problems are, and there's nothing you can do in the sub to fix this. 
@millercarbon: Ya I am not against a bass array. I would have to get probably 2 more subs and they would have to be smaller, maybe 12s or 14s because I am really running out of space. It would be great though if I could solve this without subs. I wonder if bass traps would allow me to turn my subs up more while still retaining cleaner bass.
@erik_squires : Ya moving the speakers around, forward, back, side to side seems something worth while and free! You mentioned dispersion, do you think dispersion panels behind the speakers would do anything  or be of any value ? My friend is selling two really nice 2 foot x 3 foot wood panels that look extremely well made for $400 usd for the pair.
Hi OP:

Use the tools you have. Move the speakers back to see if you can find a better bass balance. Next, see the tooth like ridges in the high frequency? That’s evidence of regular reflections, possibly even slap echoes.

Yes, diffusion is a great idea, but at $400, I suggest you call GIK Acoustics and see what they could do in your budget. I mean, I am sure these are find diffusors, but for maybe a little more you might be able to get a lot more room treatment from GIK. They have an online service where you can send them pictures and room measurements. I strongly suggest you take advantage of it.

So, this is the order I recommend you work:

  1. Main speaker placement
  2. Room treatment
  3. EQ
  4. Subwoofer

For 3, the Schiit Loki ($149) may be just perfect. Only after you have sorted this out should you spend too much time attempting to integrate your sub.  I can't tell you how much more important 2 is going to be though. 

Again, what speakers are you using though?  This may provide clues.

Best,

E