Before suggesting this I admit this is not the most solid of fixes and many here will most likely want to spank me for the suggestion. That being said I have found that if I set the left speaker, the one with the boundary gain issues, in my Sunfire Pro. at one DB less during the set up mode that some of the boundary gain goes away. Again not the best of solutions, but short of another room to place the system in or a new home I am limited as to what can be done. If your Cambridge allows you to set speaker levels during a set up process or program give it a shot, maybe you will see some improvement as well.
Thanks for sharing. Just my humble opinion but get the 1.6's out from the rear wall further, the dipole effect they are famous for is most likely suffering from being so close to the rear wall. Also not sure by looking at the picture but try a little toe in on the speakers, should help with soundstage. I have the issue of boundary gain as my left speaker is just too close to the wall, you may have have some of the same with the right speaker being so close to the wall. Try some basic acoustic wall treatements, large rugs and anything to break up the large wall surfaces seen in the pictures. Best of luck.