A gain is not a volume control! Aim for something around 450w rms as figures are quoted at 14.4v which you wont see.
that still makes no difference.
the other thing to remember is that the FLi range is a budget range and aimed in the marketplace for mainly novices. this in mind they would need to underpower the amp to sub to make sure that when the user sets it up (and lets face it normally a young lad who wont know what he's doing) he cant blow the voice coil inside by sending too many watts through the sub.