PROAC D48R Bass Booming

I had Proac Response 5's and D30 R's and had similar problems with too much bass and my room was much bigger than yours. The two things I did that helped were 1, run the speaker cables to the top binding posts (of the speaker) and jump the bottom. This will give you less energy to the woofers. If this method removes too much bass you can try a diagonal jump which is half way between the two methods.  2, if you use Roon, use their EQ function and cut the deep bass by however many db's it takes to reduce the excess bass. I first measured my room and found out at what frequency I had excess bass....Hope this helps.

I own older Proac 3.8's, and know for a fact they do not work well in a room that small. You should try stand mount monitors with a sub unless you want to go with digital room correction.

To to a local @home, Hobby Lobby or other store that sells artificial plants.  Buy four of five large, leafy artificial plants and position them along the back wall, and between and behind the speakers.  Don't be afraid to use 6-foot or taller plants.  These will act as diffusers and help to reduce the boominess.  Add or remove the plants until you get the correct balance.  

I own a pair of D30R's and I experienced the same "bass bloom" that you have.  My room is about the same area.  Speaker placement is most important.  You have a good distance from the rear wall but are maybe too close to the side walls.  I am fortunate enough to have a few amps (SS and tubes) to alternate before making a new purchase.  I found placement and footing cured the problem without major expenditures.  Good luck.

I have a friend who have the same problem with
his proac d40r, his temporary solution was to put towels
below the speaker port, essentially just laying the towels
below and not stuffing the port itself. He said it minimized
the booming.