Replying with a solution to my own reply here. Hopefully this will help someone else out. Turned out to be a weird one...
My loopback issue was apparently being caused by a USB cable I had plugged into my KSP. Now mind you, that USB cable wasn't plugged into anything on the other end, just into the KSP. I sometimes use it to plug into my phone, so I can trigger samples from the Koala sample app (which is pretty awesome) on my Galaxy. As soon as I unplugged that, the loopback stopped. And I've even got my channels set to the same thing for both in and out. Crazy.