I don’t remember what you are using, but judging by the photos you have some pretty serious open room volume. Multiple subs will make a more even bass response in the space. The size of the sub and it’s output should match the space. (Read the Bassaholics info here at AH as a good primer.)
In short, if your sub is slightly underpowered for the space and you are running it hot to compensate, I would expect you might be able to start noticing it more.
Adding subs: they should ideally be matching subs, and better would be matching subs well suited to the space.
There is a lot of opinion and personal preference for placement. I subscribe to the school of using the sub crawl to find the best acoustic spots in the room, preferably asymmetric placement of multiple subs, then backing those up with measurements.
Others like symmetry and front wall loading, corner loading, 1/4 wall, 1/2 wall... etc. I prefer my subs be away from my mains.
Many ways to go about it. In the end, learning about your rooms acoustics and working with them will get you further.