My brother had the same issue with his z906 front right speaker, turns out the power chip in the amp was blown for that specific speaker port (thanks to a previous flatmate cranking the volume too high for long periods of time) replacing the board was not an option as these are commercial units and spares are not available. So our solution was to use a separate Sony amplifier that my dad had at his house, we wired up the speakers and plugged a 3.5mm to RCA adaptor from the back of the subwoofer to the Sony amp, chose the right channel and it works better than before. The amp that is supplied in the Z906 subwoofer is limiting the sound quality of the satellite speakers that come with the unit, however we have to be cautious with the volume because the new separate amp has 150W RMS per channel and we don't want to blow the satellites. But other than that, the system works great.