I would suggest that you program the 861 to go to an empty digital input when SLS is selected so the rear speakers have no signal. I believe that should do the trick

John

Main system: Cable, Apple TV, HD621, G68D, AC11, 218, DSP7200SE (L+R), DSP3200 (L+R Rear)
2nd system: Cable, Apple TV, MC200, HD621, 568.2, M3100 (L+R)