Welcome, mr_rodge!
The speakers will be fine for your room. Also, your receiver is fine for the M3s. They are a relatively easy load and should pose no problem. You will not damage the M3s unless you crank the volume ALL the way up.
As for the subwoofer, it can definitely add to the bottom octave. Don't worry too much about overpowering bass as you can adjust the gain on the sub to play as loud or as little as you want. If you are happy with the performance of the M3s by themselves, don't fret too much about the sub. It's all about preference. The M3s should put out a fair amount of bass in a room your size.
