For a stereo signal to be sent to the subwoofer from an AVR, the front speaker size must be set in the set up menu to 'small' rather than 'large'. This setting is not indicative of the physical size of the speakers, but rather it determines how the units bass management occurs. With the front speakers set to 'large', all bass will be sent to the front speakers, and none to the subwoofer, irrespective of whether a subwoofer is selected as 'on' or 'off'. When the front speakers are set to 'small' the bass is sent to both the subwoofer and the front speakers.
Note: the above only applies to ProLogic or stereo signals. The LFE channel (the .1 in a 5.1 system) of a Dolby or DTS soundtrack will always be sent to the subwoofer, regardless of speaker size settings.