generally the HT bypass functions as nothing more than a fixed level output operating at unity gain (bypasses the volume control). this is not strictly necessary, as any input will work in the same way if you're willing to run the volume up each time you select the input.
in your case, just run the main L / R outs from your HT processor into any input of the C15, run the volume up on the Mac pre, and match the levels using the HT processor.
just make sure you turn the volume back down on the C15 before switching to another input!
in your case, just run the main L / R outs from your HT processor into any input of the C15, run the volume up on the Mac pre, and match the levels using the HT processor.
just make sure you turn the volume back down on the C15 before switching to another input!