Question regarding subwoofers

The one thing that really matters is the one thing you don't mention: crossover frequency. It should be below 100Hz, and 80Hz is even better. 

This is because bass this low in frequency, you cannot localize it. If a sub is putting out any energy above about 100Hz you will localize it and that is why your bass seems to be coming from one spot. 

Below 80 you can't tell where the sub is, you can only tell there's more bass. Where the bass is coming from, your main speakers provide those location cues. Also since you can't localize there is no L and no R.

Look at my system. https://systems.audiogon.com/systems/8367 There's five subs. They're spaced asymmetrically around the room. There's one up front that's left of center, and I never have any idea its putting out any bass at all. Never can tell any of them is putting out anything. At all. Only hear plenty of good clean clear articulate dynamic and extremely deep bass. In spite of all the different drivers and cabinets and locations the bass is beautifully integrated seamlessly into one enveloping sound stage.  

So your solution is: 
Move the center sub to near one corner. Set it to 80. Adjust the level. Buy as many more subs as you can, put them near to - but not all the same distance from - the other corners. Adjust them all. Enjoy better bass than you ever heard before. 

Its kind of a new thing. Been around 20 years but news travels slow. Do a search- Swarm subwoofer system, Distributed Bass Array. You will see.

To get a reference point, listen to jazz with the subwoofer turned OFF. Is the bass still centered? If so, the sub may NOT be to blame. Regardless, the sub crossover frequency should be set no higher than 80Hz or bass localization, as mentioned by MC, will result. If you know the LF -3dB of the Acoustats is, that's where I would set the sub crossover frequency. Then, it's simply a matter of adjusting the sub gain to suit your listening tastes.

As for new subs, the number and size would depend primarily on room size and listening preferences. I would work with what I had before going that route.

If you can tell where the woofer is, its crossed over too high as @millercarbon points out. If it does not make any output above 80Hz it won't attract attention to itself. Your Acoustats go lower than that with ease, so this should be a simple matter of resetting the crossover point.

The distributed bass array is a good tip.

+ 1 @turnbow, adding; a sub enclosure with two drivers ( 8" for example )
will displace as much air as a single 16 " driver.
Larger Drivers are heavier and benefit more robust motors as would any driver.
Servo feedback subs tend to be accurate IMO.

rego199 posts10-28-2020 3:00pm
"....a sub enclosure with two drivers ( 8" for example )
will displace as much air as a single 16 " driver...."

That’s not quite correct. An 8" driver has a cone area of 50 sq.in so two 8" drivers would equal 100 sq.in. A 16" driver has a cone area of 201 sq.in. Cone area varies to the square of the radius.