Another option for outdoors is multiple single 18" subs. This way you can use 1 pair or 2 pair depending on the need (and can also start with a lower cost). Tops need to be good though, as you really can't double them up without issues unless they have narrow dispersion horns, which then limits their use indoors. If you have a seated audience, and the space is deep, you can also use a pair of speakers halfway out on a delay to help reach the back of the audience. Doesn't work as well if they're moving around though.
Good outdoor tops are the QSC KW (or older HPR line used) , JBL SRX, EV Tour X, and I'm sure there are others.
Good outdoor tops are the QSC KW (or older HPR line used) , JBL SRX, EV Tour X, and I'm sure there are others.