I am not familiar with the operation of the unit, but a coupe of potential things are that it could be the remote is faulty and turning down the volume, so I would try removing the batteries and seeing if the amp still does the same thing. The other could be that if playing at high volume it’s...
Plugging in headphones mutes all other outputs on my CXA81, which suggests that you may have a fault in the unit.
As per the Manual:
Note: Plugging in headphones will automatically mute the loudspeaker, pre-out, and sub-outputs.