Surely the whole point of an FRFR setup is that has the full range for your purposes, if you need to add a sub for head room or range then it hasn't got enough usable range and you've bought the wrong kit, when you consider a pair of RCFs can handle front of house for a full band including keys and kick drum for a medium sized venue then a single one as a monitor should cover every gig, anything bigger would require wedges around the stage or in ears.