Jim, I'm hard-pressed to think of a musical passage that is more likely to bring out a marginal volume range/system gain condition than the finale of the "Firebird," due to its incredible dynamic range. Perhaps that is why you chose to listen to it in your initial evaluation.
I have on LP the digitally recorded version of the Suite that was issued by Telarc ca. 1980, with Robert Shaw conducting the Atlanta Symphony. Like most or all Telarc recordings it was subjected to little or no dynamic compression when it was produced, the result being that the whisper-like level of the opening notes will tempt one to turn up the volume control higher than on pretty much any other recording I can think of. While some of the notes a few minutes later, especially the one at the very end, will reach or exceed the maximum sound pressure level that can be produced by many highly regarded systems.
Depending on how much dynamic compression was introduced when the version on your sampler was engineered it seems conceivable to me that both of the factors you mentioned, plus the dynamic range of the recording, plus the possibility I mentioned earlier that the unspecified gain between the Sphinx's line level inputs and its variable line-level output may be little or none, may all have contributed to the fact that you max'd out the volume setting on that recording. Hopefully that will not happen with very many of your other recordings, until such time as you purchase a preamp.
Best regards,
-- Al