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

Your room is absolutly unique regarding the positioning and integration of a subwoofer/s. Other people's dimensions and locations, even if they're identical or similar, will require a great deal of trial and mostly error.

If you locate a sub at the listening position by using long enough interconnects (Monoprice or Bluejeans Cable) and an extension power cord while playing low frequency tone/s will allow you to walk (or crawl) around your room and map out YOUR rooms modes and nulls. This test tone download should be useful https://realtraps.com/test-cd.htm

This method is simple and need only be done one time. Once you've mapped YOUR room you can experiment by placing your sub/s at or very near the mode (louder bass) areas.

I found multiple subwoofers evened out my rooms modes. Controlling the subwoofers frequency response is a product of equalization and room treatment (Eric's blog). No amount of subwoofers can control frequency. While using four large subs I found equalization absolutly made such a desirable improvement I found two small subs to be sufficient. Then again, along with personal taste every room is unique. 
This is well worth the effort good luck with it. 

Thanks guys for the great thoughts.
Erik, sounds like a good plan.
Miller, should I still use my 18s with the 10s? Like 2 18s and 2 10s?
Strojo: Via RCA from preamp to subs.

Absolutely. Notice my system https://systems.audiogon.com/systems/8367 There's a 12" isobaric Talon Roc sub in the front left corner, two sealed 10" subs towards the front on the left and right, and two 10" ported subs in the back along the L and R walls. Each sub is near the wall but they are all different distances from the corners.  

The response of each of these subs is different, and the locations are different, and they even are adjusted somewhat differently. The Talon is powered and so its independent. The left two are on one Dayton amp, the right two on another. Left and right are close but not identical in level. Or phase. All this asymmetry would be a nightmare for the mains, but is actually what you want with subs. All the slightly different responses produce different modes, in different places, and at different levels.  

Placement becomes a whole lot easier with more subs. With just one you have one or two nasty humps to figure out. With four each one plays at a lot lower level so the humps are much smaller, and you can put them just about anywhere you want. I tried a lot of different things because it mattered so much with just the one. But with five I find it hardly matters at all.  

Hook em all up, you will see. 

That is so fascinating using all different kinds of subwoofers. Is there any downsides to this? Do they have different speed or timbre?

Right now they are about 10 inches off the wall and they probably need to be moved out more.

@smodtactical  If your bass is rolled off, you're usually better off moving the speaker closer to the wall behind it. This is particularly true in the case of a sub- as it gets closer to the wall, the boundary effects start to come into play. So the bass goes up not down. But standing waves will also be more pronounced, so you should try the sub pointed in different directions, with different phase and in different locations along the wall if you can. The right spot might only be a few inches wide so if you're playing with a single sub this requires patience!