I have the Onkyo SR-TX703, which is pretty similar to the 805. According to the manual, in STEREO mode, sound is only output by the front L & R speakers. To get the sub to come on, you will need to select a DSP mode like DPL-IIx music. But this will also send output to your surround speakers.
If you want to use the subwoofer with STEREO mode, you will need to run the sub off of the front L & R speaker outputs.
TIC
If you want to use the subwoofer with STEREO mode, you will need to run the sub off of the front L & R speaker outputs.
TIC