When you adjust accompaniment parts in the main mixer window, it adjusts the volume balance throughout the entire performance / style.

Hi Peter
One way to get round this problem is:-
Program Menus>Composer>'A''B' or 'C'> Intro>OK>Bass(or any part)>Part Setting>then change the 'Balance' number. 0 is the loudest setting and -127 is silent. You can alter each or all parts this way. It's a bit trial and error but it doesnt take too long and providing you save the composer style, you'll have your edited intro (or whatever) within that style whenever you need it.
John