Yeah, this is a very common issue with the "Auto"-sensing on most subwoofers. At low volume, it just doesn't recognize the signal coming from the receiver. The simplest solution is just to leave the subwoofer in the "On" setting. With nothing playing, it doesn't draw very much power at all - maybe 3-5 Watts. Although that's still higher than what it draws on "standby".
The other solution is just to turn up the volume loud enough for the subwoofer to recognize the signal and turn on and then back the volume down to where you want it. Some subwoofers will turn off after a while if you do this though.
You could try turning up the subwoofer trim level (that +4dB setting) to its maximum on the receiver and then turning the volume knob on the subwoofer itself down to compensate. That might give the signal from the receiver enough strength to activate the "auto on" sensor even at lower playback volumes. I can't promise anything, but it's worth a try
Honestly, I messed with the same problem for a little while, but then I just went the easy route and leave my subwoofers in the "on" position at all time
