Well, sorry for that! You're probably right, but this seemed like something that could cause damage if set wrong and you can find all sorts of things on the Internet.
Manual is not so clear on this. It lets you choose while the manual from the speakers names two values: 8Ω (min. 3.2Ω).
To me it looks like one of those; "you can choose ANY option, ANY indeed (but one will fry your system)!! Go ahead, choose!"