Looking to upgrade my back surround sound speakers

I am looking to upgrade both my back surround sound speakers as part of my 5.1 setup. I currently have Martin Logan Spire as my L/R and the Martin Logan Encore as my center channel. Any suggestions? Should I keep with the Martin Logan brand...do they make dipole/bipole/monopole speakers?

I have a beam about 10 inches behind the listening area and a wall in which I would mount the rears. I prefer not to dig into the wall and thus prefer mounted speakers.