Playing music is a normal stereo PCM signal which is sent to ur avr. When a song is recorded at the studio, they adjust the volume so that the loudest signal is at 0db without getting distorted.
Movies in DolbyDigital, DTS, etc. are not recorded and adjusted this way. There are quiet dialogs, much quiet background sounds, and there are loud action scenes. This is made to achieve a more realistic sound and is called the dynamic range.
XBMC doesn´t behave diffrent than any other standalone BD player when bitstreaming.
if u enable dynamic volume, the dynamic range get compressed, so that the quiet dialogs become louder and the heavy action scenes become more quiet. This might be good work around for watching movies at night if the kids are in bed or u will not stress ur neightborhood, but its not the way a movie should sound and it´s killing the hole atmosphere in a movie.
this thread makes me believe that u have never listed to a movie in real Dolby/DTS with its full dynamic range....that´s a pity
the reason zoom and vlc are playing at a louder level is, that these players are also configured to send a pcm stereo signal to ur avr.
edit: and the option for amplification isn´t available, cause u bitstream the audio to ur denon. There is nothing ur htpc will do with the sound data when bitstreaming, and u have no chance of adjusting volume. Turn up volume on ur denon as suggested. This the right way.