For those of us just getting to grips with the finer intricacies of Cubase SX, the following might be of interest.
Making GPO parts louder and softer using the Logical Editor.
1) Select a part
2) Right-click -> edit -> Logical Editor
3) Create a new logical edit of type Transform.
Top box rules are
Type Is : Equal : Controller : AND
MIDI Controller No. : Equal : 1
Bottom box rule is
Value 2 : Subtract : 20
4) Store preset as "Quieten Modulation"
.. then hit DO IT to apply it to the part. You can also create one that does Value 2 : Add : 20, store as "Louden Modulation". When you need them again, select Quieten or Louden from the preset dropdown.
You can also multi-select many parts (drag a box round the clips, or shift click to multi-select) and louden a whole passage in one fell swoop.
I've also bound this to the 'e' key, so I can select a whole passage, hit 'e' and louden or quieten it quickly.