What would need filling in? Below 16hz?
Those nasty suckouts. If you place the sub closer to the listening position and further from a wall/corner then you can hope to provide some "fill" where the main speakers produce a terrible suckout at your listening position. It won't completely cure a suckout but along with bass trapping it may help just enough to make an improvement. In this situation with full range speakers one could easily end up with overall too much bass - this is where a PEQ with notch filters can help on the sub to avoid adding to both peaks and troughs...you just add to the troughs and the sub peaks are EQ'd down. Of course you could notch filter the whole lot (mains and Sub) with a PEQ also (depending on the specific issues) or you could EQ each speaker and sub seperately...indeed the possibilities are many with the only caveat being that it is all too easy to overdo things and try to get ruler flat by placing dozens of filters which I suspect is a bad idea. At some point diminishing returns and the KISS - Keep It Simple Stupid rule of thumb applies....less "tweaking" may actually be better sounding than more.