Well, it was a nice theory but didn't quite work out. I tried disabling just the regular l/r surrounds in my 5.1 setup via the Speaker size menu in the OSD and the has the affect of also disabling the surrounds via the 7.1 inputs. The toggle set to either 'hpf/lpf' or 'bypass' didn't make any difference, in both cases there was no sound from the surround speakers if they were set to NONE in the OSD. I'm assuming the SBL/SBR speakers would behave similarly, that is if they're disabled in the OSD there is no output from the speaker terminals even if 7.1 input is selected.

-John