I'm sure others will have better and more accurate information for you but you could make a dummy load for the unused channel. Use a matching impedance resistor of the largest wattage you can find that is at least 5 watts since the max you should get from a single channel is 2.3 watts; so 8 or 6 or 4 ohms (matching your Fortes) at 5 watts wired across the unused channel might/should work. Not sure it would hurt to have the channel unterminated if you have no signal on it but I'll leave that to others to expound on.

You should double check this reply but I think Steve recommends running the output shorted as opposed to no speaker load. I read that somewhere in one of the many owner's manuals but I don’t recall exactly where.