Some movies have more dynamic range than others. During dialog, 5.1-channel sources can sound like they're a lot quieter than 2.0 sources (including ones that are downmixed from 5.1), but in reality, the peak levels are the same while the average levels are different. If 5.1 soundtracks sound pretty quiet during the dialog, when the sound effects start you can still wake up the neighbours. The preamp just plays what it receives from the source.