Hi rictuga,
Welcome to the PSR Tutorial Forum!
This parameter is called "Volume(MIDI Song) Offset". As can be seen from the SX900 Data List (chapter "Parameter Chart"), page 87, this parameter can only be saved in Registrations.
Many users use one or more "Setup Registrations", of which the user simply loads the appropriate one immediately after starting the keyboard. So simply create a Registration that, in addition to your other preferred settings (e.g. fingering, split points, pedal settings), also contains an adapted MIDI Song(Offset) Volume. It is important that when saving on a Registration button next to the other required checkmarks (e.g. Style, Voice, Keyboard Harmony/Arpeggio, Pedal) the checkmark for "MIDI Song" is also set.
Of course, the Registration BANK must then also be saved as a file, as usual. It is best to use a special character (e.g. an underscore, an exclamation mark or an apostrophe) at the beginning of the file name of the Setup Registration Bank, so that this file is dispayed at the top and can be found quickly.
Alternatively you could also call up ALL of your MIDI songs via Registrations (preferably using one Bank file per MIDI song). Then you could save a different, suitable Offset Volume for each MIDI song if necessary.
By the way: I use the former method (on the Genos and Tyros5), i.e. I set the MIDI Song(Offset) Volume to 70 at the beginning of a session or a performance (via a Setup Registration or sometimes simply manually), and that's all. (I usually load MIDI songs directly, not via Registrations.)
Hope this helps!
Best regards,
Chris