Yes you could place 518 between 500 and 568. You could also connect the optical output of your HDMI extractor into 518, if you dont play much 96Khz music on Amazon. The 518 is limited to 24bit 48kHz.

The way I would describe what the 518 does, would be that it "Meridianizes" any input you connect into it, as in its most basic function it de-jitters and resolution enhances the input.

It will also add to the inputs of 568.

Have you read the 518 manual?