We have an issue with the FM radio on our ZX600. After listening to a station for more than an hour, or so, the sound gets very muffled, boomy and distorted. Otherwise the unit works fine when listening and watching TV and DVDs for an extended period of time.
We are camp hosting right now and I did find that you can overdrive the amplifier and get very distorted sound if you have the volume too high coming out of the TV. I suspect the sound levels can also be adjusted using the volume control on iPods. Once I hooked up my iPod and noticed that I had to turn up the Concertone 's volume over 30 to get satisfactory sound. Probably just needed to increase the output of the iPod to get a good input volume.