Not sure if this is even an appropriate place to post all this. I'm just going to charge ahead anyway in hopes someone will eventually see it.

I just did a test of the inputs by plugging a simple stereo mini patch cable from the rear-speaker outs into each of the inputs in turn - both line in (blue) and mic in (pink.) I set my default system output to rear speakers. I launched Peak Pro, and told it to record from blue and pink in turn, and to play out the main speakers, which I then plugged headphones into. It all worked perfectly. iTunes played out the rear channels, and Peak was able to record perfectly from both inputs, and play to the main channels.

So, I now have four output channels and four input channels. Not bad.