I assume you have tried to fiddle with the preferences for AirTunes in:
Preferences > Advanced > General Tab > middle section which describes how iTunes handles sound through Airport.
I have BL3's connected to a BS9000 to which an AEXpress is connected through AUX input, and I have had no volume abnormalities.
Please note that my preferences are as follows (your results may vary):
Streaming buffer size: Medium
CHECKED: look for remote speakers connected with AirTunes
UNCHECKED: Disable iTunes volume control for remote speakers
CHECKED: Allow iTunes control from remote speakers
Hope this helps.
-Sal
Love B&O, but no longer addicted.