Hi Graham,
I don't own a DGX-670, so I can't try the following myself, but I checked the manuals:
In the Mixer display you can set the Volume of the entire Style in the "Panel" page. In the "Style" page you should be able to set the Volume for each Style part individually.
In my opinion, it is easiest to save such changes directly in Registrations. Since the DGX-670 has a Style Creator, you should alternatively be able to save these changes as a new STYLE (call up Style Creator > confirm to edit the currently selected Style > SAVE). Have you tried this yet?
Best regards,
Chris