IMAY I tried everything as you wrote.  center speaker, turned off the subwoofer, then soon 028=01 nothing at all on that setting.  then 028=04 there was a weak sound on that setting.  but the volume on the ihu was at high max.  balancer and fader in all positions.  without change.  despair.
normal sound on channels 1 and 3.
2.4.5. quiet weak
	
	
	
	
normal sound on channels 1 and 3.
2.4.5. quiet weak

 
 

 
