What 18" driver suppose for home theater

Hope you help.
Now need 18" driver for making home theater big sub. Two sub actualy.

Front speaker will be with 12" eton driver, it go down to 35Hz/
So I need subs with go lower down to 20Hz, will be good :).
Here in Ukraine available brands B&Cspeakers, RCF, have two used JBL2241, maybe some thers.
Hope you suggest goog sub driver for my home theater project.

Have ability to make big box up to 250l no problem. Class amplifier 750wt or 1400wt.
Budjet no problem, up to 700usd