speakers for marshall superbass

hello to all
i own a couple of superbass (76+78) and i am going to build a cabinet for
i have no experience with bass speakers and would like to know which models
could work with my amps. i have a jcm800 bass series cab, but the spekers in
it are the g-80's, exactly the same of the guitar cabinet, and i am worried
about them, they dont give me confidence. i am oriented to jbl k145 (15") or
something like that, but are diffucult to find.
could you suggest something else?
greetings from italy