just a quick update:

I tried a reset again yesterday. After resetting I left everything at default. Ran a test tone and the subs worked. Tried a few sources and nothing.

I changed all the audio inputs from HDMI to either coax or spdif and tried a few random sources and the subs worked. At this point all the listening modes were in Auto - which isn't really covered in the manual so I can only assume how it works. I reset the unit again and decided to run the auto setup feature instead of entering my own measurements. When I did that, the test failed. The test failed because I forgot to tell the 976 that I do not have SBL/R speakers. I went to speaker setup and set the SBL/R to none and re-ran the auto setup. Worked as it should and set things up very close to what I had done manually.

I set all the listening modes to PLII Movie and tried a few sources and everything seems to be working as it should.

Now I'm wondering - when I reset the 976 a week after it came back from repair, did I forget to set the SBL/R to none? If I did, would having the listening mode set to auto have some impact on the LFE signal?

It doesn't explain why I wasn't getting anything from the test tones unless the volume was too low, but I can remember what that was - just that there was nothing when touching the cone.

Things don't miraculously fix themselves so either it's an intermittent problem that hasn't been repaired or through the several resets I attempted I managed to find a some bizarre combinations of conflicting settings. IDK. Glad it's working again.

Can anyone explain exactly how the Auto listening mode works? My assumption is that it switches to what it senses - some variant of Dolby or DTS - or it makes 2.0 5.2 or 7.1. But I could be wrong. It's happened before.