From my experience also with speakers much bigger than the FR 30, the fact that they go down deep doesn’t mean much in practice. Reason is, that it would be pure luck with very low chance, if they would cover the area of the frequency needed, at the place they are positioned for soundstage reasons. Even if, they’d quite sure produce a noticeable suck out at another frequency around.
Even if you’re able to flexibly position a sub in the room, you’re only able to flatten its response at a prioritized frequency +/- 5-10 Hz. To get it flat enough in its whole spectrum, the sub must additionally be EQ’able. Otherwise you have to work against remaining peaks with decent room treatment.