Okay, so I don't own the A-9555 but the A-9755, but my problem (if it is a problem) is probably common to both amps.
When I'm not playing music through the speakers I'm hearing a slight static hiss from the treble when I put my ear close to the driver. The hiss increases when I turn on loudness. The hiss is not audible from the listening position, only when you listen close to the treble driver, so it's not a problem really. I'm just curious whether they are all like this or if my unit is defective.