I've used Sony receivers in the past, and, even though there have been an extensive list of discrete commands for other channels' levels, I've never seen any for the master volume.

That receiver does have an RS232 port, and it's possible that discrete volume commands exist for this, but finding, and understanding, the Sony protocol manuals may be difficult.

If your problem is between analog and digital sources, you may be able to adjust the analog line levels to better match the digitals.

[Link: amazon.com]

[Link: rdlnet.com]