Well, I don't own an S-970, but I do own an S-950, which has pretty much the same attributes when it comes to recording.
First and foremost, set the master volume of the keyboard to about the 12 O'clock position - that's all you really need and anything above that threatens distortion.
Next, the default style volumes seem to work best for most USB recordings.
When you fire up the USB recorder, set the recording level at 70. Yeah, I know that sounds low, but it keeps those recordings crisp and clear.
After recording the song, remove the USB stick, place it in your PC and fire up Audacity. When you get to Audacity, select effects and set the Normalize setting to -0.6 db, which is where most songs are set at for publishing.
It's that easy,
Gary