My recollection is that most believe there is a very noticeable improvement going from 2 to 3 subs and a bit more going to four, but after that pretty much diminishing returns.
Todd Welti is The Man when it comes to multi-subwoofer system set-up and his Harman Kardon article is well worth reading. While there is a bit of science to it, you can also do pretty well by randomly placing a sub along each wall. I strongly recommend using REW to help guide your replacement. Sometimes the flattest frequency response worsens room modes, which I think are far more onerous.
I have a version of the Audio Kinesis Swarm and the benefits are significant. My system pics graphically show the differences between subs on and subs off for both frequency response and room modes. My system is all analog and I imagine DSP could flatten it even more. Imaging and depth are also improved and even more so if you can go stereo.