Okay, I understand. To have fun from a nearfield monitor has probably more to do with playing your mix on other systems and get happy by the fact that your mix sounds good almost everywhere. (in other words, that your nearfields translate well to other systems)
So if i understand correctly the extra sub is more for a surround sound then the extra need for more/clearer bass? I imagine that a 7"inch woofer should go pretty far into the low end for mixing purposes doesn't it?