The original designs of the RSP-1066, RSX-1055 and RSX-1065 did not allow this to be done, short of turning off the subwoofer in the Speaker setup menu before using 2 Channel mode. However, the ability to disable the subwoofer output for different surround modes, including stereo, has been added in a later software update for the unit. 

With software versions listed below and beyond, the subwoofer output can be disabled by accessing the SUB SETUP menu from the OSD, and for each surround mode the subwoofer level can be adjusted to +10dB and –9dB.  Lowering the subwoofer level below –9dB will display OFF, and will disable subwoofer output for this DSP mode.  Using this method, the subwoofer output can be separately enabled or disabled for 2, 5, and 7 Channel Stereo;  the 4 Music Modes; all Dolby and all dts modes.

• RSP-1066: V1.48 or V2.24 and later
• RSX-1055: V1.48 or V2.24 and later
• RSX-1065: V1.55 or V2.22 and later