That's enough resets for sure. The thread is getting to long to trace back if you have tried the following:
1) Go to speaker menu, level and adjust SW1 to +10 and use the test tone to see if the sub now sounds louder.
2) If that doesn't work, use analog input (ext. DAC, CD player etc.), use stereo mode with the sub connected to sub1, and on the audio menu select stereo with sub on, and see what happens.
Just trying to see such changes can make a difference and narrow down the issue, not to fix the issue as such at the moment.