Welcome to the forum, Craig!
Hopefully someone with experience will answer you, but I'll throw my thoughts in...if only to bump your thread and welcome you.
My guess is that HIN is for high-frequency input, and LIN is for low-frequency input, and that the two sets of binding posts allow you to biamp the speakers. Is there a wire or metal bar connecting the two "+" terminals, and another one connecting the two "-" terminals?
Just like you, I checked the manufacturer's website. The owner's manual doesn't give information about the speaker terminals, so I didn't even see a diagram of them. The manual does give a technical support phone number to call, though, so maybe try that number if you don't find the answer.