Wall mounting a pair of Monitor Audio BX2s

Have not tried them but as they have a front bass reflex port they should not be affected as much as a rear firing ported speaker.  However, even front ported speaker too close of a rear wall can result in a little booming (dependant on the individual speaker) but I believe that they are supplied with port bungs and using those may help reduce that.  Would suggest that you try them close to rear wall unbunged & then bunged and see if that helps.

You may even find that being close to the wall improves the bass...it is all down to what you like so experiment (but not to the point of drilling holes in walls just to test).  In the past I have managed to rig up something suitable as a temporary measure prior to commiting to positioning.