external speakers not working


I just got an iMac G5 with OS X Tiger installed. When I plug in my headphones, everything works fine and OSX switches the sound over to headphones as the output. However, when I try to plug in my external speakers (hooked up through an amplifier with audio in), the OSX switches the settings to "digital out" and there is no sound (checked everything from the mute buttons to the volumes). Is there any way to force OSX to keep the headphones setting as the output so that the amplifier will work? It all works perfectly with windows, linux and game systems, not sure why my mac is being difficult. Any help would be greatly appreciated, as I love my new mac (with the exception of this minor issue)