I know in theory that 7.1 adds an extra speaker to the surround back channel in a 6.1 setup. The source itself is mono. My question is in the wiring of the extra speaker, should I wire them in series or parallel? Or is it better to get self-powered speakers and use the pre-out direct inputs that allow for a seperate sbl and sbr sources? Also what 4 speakers would you recommend for this setup?