Unless it's broken it's probably your fader adjustment.
In the old days there woud just be a knob for this!
Adjusting the sound characteristics — BAL/FAD/SUB
You can adjust the balance, fader, and subwoofer
volume.
1 Press (SEL) repeatedly until “BAL,”
“FAD” or “SUB” appears.
The item changes as follows:
LOW*1tMID*1tHI*1
BAL (left-right) FAD (front-rear)
SUB (subwoofer volume)*2
*1 When EQ3 is activated (page 9).
*2 When the audio output is set to “SUB” (page 10).
“ATT” is displayed at the lowest setting, and can
be adjusted up to 20 steps.
2 Press the volume +/– button
repeatedly to adjust the selected item.
After 3 seconds, the setting is complete and
the display returns to normal play/reception mode.
Note Adjust within 3 seconds of selecting the item.
Be quick, only a 3 second window to make the adjustment!