That is a TOUGH room size, about as bad as it gets for taming room acoustics and nodes. You're going to have a heavy bass node around 63-65hz, and a big suckout around 95hz, then another node about 125-130hz. Add on top of that early high-amplitude reflections from all walls, and you have a difficult setup. Square rooms essentially double the size of nodes and nulls because of being equidistant.
Your only hope is to treat every wall. I would start with something like GIK tri-traps in all 4 corners of the room, floor to ceiling if you can. Then treat side walls with 2" thick panels to tame reflections, treat behind speakers on the front wall with either 2" or 4" thick panels, and then diffusors on the back wall behind your seating position. Fortunately it's small room which will keep costs down. Also put down some thick carpet if you don't have it already, and use a fabric chair or couch for your listening seat (not leather).
Don't think about buying new speakers at this point because big or small, the room acoustics will impact them the same way. Without doubt the treatments will improve the situation considerably, but impossible to know if they'll improve it enough for you to be happy until after installation.
