Hi Mark,
The settings for volume in the Composer are relative. They are expressed as a negative number in the Part Settings of the Composer. So, let's say you have your bass part set to a Volume of 105 on the main screen or mixer and the Balance setting of the bass in the Composer is set to -15 then the actual resulting volume is 105-15=90. This balance can (has to) be set for each part of each intro, ending, fills and variations individually, 20 in all (which is quite painful when you want to edit them, lots of edits to tweak an entire Composer!).
There is something misleading when Composer editing... you can adjust the volumes but that is only for auditioning whilst you are editing. They only persist if you edit the Balance in the Part Settings. More's the pity!
Best wishes,
Mike