That was under Parametric Equaliser. Add filter - Remove high frequencies - apply to Subwoofer channel. Frequency 105Hz, slope = 24dB/octave.
Output Format Subwoofer as well as Room Correction Subwoofer settings are all greyed out due to "No upmixing or downmixing" selection under Output Format.
Edit:
I have isolated the combinations that cause problems!
1. SoX resampling causes the subwoofer channel to go to zero! No doubt of it. As soon as you turn it on, the problem of 0.00dB returns immediiately.
Then with SoX turned off:
2. In Parametric EQ, an individual filter applied to the sub channel doesn't seem to bother things too much. I got one sub drop-out yesterday that was corrected by pressing Stop then Play again. With SoX on, a restart of Media Server was required.
But with SoX either on OR off:
3. I have a series of PEQ filters applied to the Left and Right main front channels for in-room bass node suppression. If the sub channel is added to these same filters (i.e. apply them to 3 channels instead of 2), then you get those crazy negative thousands of dB showing in the sub channel of the Analyzer display when a track
not containing a sub channel is played, and
4. The application of any low pass filter in PEQ (even with no other filters shared with other channels) to the Sub channel can cause that channel to go to 0.00dB when a 6 channel track known to contain sub channel content is played.
That'll be $1000.
Addend: Summary: As long as I leave SoX off and do not apply any filter to the Sub in Parametric Equaliser, things seem stable - so far anyway. The only problem is when starting Play of a 6 channel track before say a 5 channel track has finished. Then you can lose the sub channel. But the sub channel is restored by pressing Stop and then Play again. Seems some serious debugging is needed.