I don't know if it was mentioned, but the first thing I would do is move the sub !!!
It's often a bad idea to put it in the corner.
I realize you don't have much room but try to move sub to center of the one of the walls. I'll bet you'd see a large improvement by swapping "Printer Stands"
Try to get it away from the wall as much as you can too.
When getting a new sub or speaker the FIRST step in optimizing it is placement. The LAST step is equalization.