I use CasmEdit:

1. Open the style in CasmEdit
2. Export the style (splitting it into Casm file and Midi file
3. Open midi file in Cakewalk
4. Press select all
5. Go to MIDI EFFECTS>VELOCITY
6. Press 80%
7. Press SAVE
8. Go to CasmEdit and import files
9. Save as the same name

Now all of the tracks are 80% of the volume as before.

This is the quickest way that I've found of turning down a style.

Larry