Assuming you're going to run the automatic calibration on the Onkyo, set the
power switch to on, not auto
LF extension to 30Hz
Gain to about 2 o'clock
LFE mode to on
And ignore the Xover Freq control, as the LFE mode bypasses it. Connect the subwoofer cable to either of the line level inputs.
Now run the automatic calibration, have a listen, and then adjust the sub's gain control by ear, or using the subwoofer levels in the 806's menus, to taste.
If the bass still sounds very weak, try turning the phase control to 180 and see whether this improves things. If it does all well and good, if it doesn't put it back to 0 and crank up the gain a bit more.
When you're happy, switch the on/auto switch top left to auto, which now means the sub will power off when the system hasn't sent it a signal for a while, and 'wake up' when a signal is detected.
We only turned it off before because it can confuse the auto-calibration if the sub has gone into auto-standby, as the calibration test tones may not be loud enough to 'wake it up'.