PROAC D48R Bass Booming

Your getting some great advice here. I had a similar issue with myD-30R’s. Bass was overpowering my room and sounded a bit sloppy at times. Did the easy stuff first, trying to position them correctly.
This helped somewhat. I then replaced the stock spikes with a set of Audio Points which made a huge improvement. Not only was the bass tighter and more defined, but the mid range and top end improved as well. The D-48R’s are wonderful and capable speakers and worth the time and effort to get right. Good luck

Have you considered changing to something like a Lyngdorf TDIA 2170 or 3400. They are fantastic units and I can almost guarantee it will resolve your bass problems. The Room Correction software is really amazing IMO with no downsides. 

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.