OK we will assume there is something wrong with standby mode. That doesn't mean the subwoofer problem is a receiver failure.
From page 35 of your receiver manual.
"2CH downmix mode
• This mode allows the multi-channel signals encoded in DTS or Dolby Digital format, etc. to be mixed down into
2 front channels and to be reproduced through only two front speakers or through headphones.
• When the SPEAKER button is set to off to listen with headphones, if the STEREO button is pressed while
playing the multi-channel digital signals from DTS or Dolby Digital sources, etc., it will enter the 2CH downmix
mode automatically.
• To cancel the 2CH downmix mode, select the Dolby Headphone mode with using the MULTI CONTROL knob
on the front panel or the SURROUND MODE UP/DOWN ( >/< ) buttons on the remote control."
You might double check that you don't have 2 channel downmix mode selected.
