F3 is an industry standard that indicates where a speaker/enclosure loses 3dbs and SPL is a standard measurement of sound . I believe that a 5 string can play to about low 30,s Hz,s ? . how big of a crowd do you play for ?
They would work in Cubo 18 Extended to get you started, if you decide you need more SPL, build something bigger. Just remember these drivers aren't exactly monsters, use a 40 Hz high pass and if you need more SPL, other drivers might probably get you more.