The operative phrase was "older" not necessarily the number of channels. Older movies, like ones from the 40s, would sound more controlled dynamically because they are compressed and mixed for clarity with a more limited dynamic range. They are normalized up to peak level and this means their overall level is likely to sound louder. So you turn the volume down a bit to compensate. When a commercial comes up at peak level, it isn't a great contrast. A modern dynamic soundtrack might go to commercial at a very low level in a quiet passage, and the contrast in volume compared to the normalized up commercial would be more jarring. For low level late night listening, more compressed dynamics is preferable. If you are giving the program your full attention and the volume is higher, a dynamic program might be better. Of course commercials are always a pain.