Thank you for these comments. I spoke with the fine folks at Rogue Audio; they said to use the variable pre-out to the receiver from the Sphinx, find a level that is good on the Sphinx for CD and use the receiver for volume control. This setup is probably not as good as using the Sphinx by itself but I do gain output power and audio controls that are not available with the Sphinx.
I didn't ask but I wonder what, then, the purpose of the fixed pre-out connection might be.
I didn't ask but I wonder what, then, the purpose of the fixed pre-out connection might be.