Some speakers are grossly inefficient but sound amazing (like Maggies or some Thiels). Those are cases where even low volumes require a minimum of 10-50 watts just to "get them going". Personally, I'd want hundreds of watts with those two but, on the other hand, I'd have a hard time adjusting the volume on a speaker rated at 102db like coincidents or klipshorns.
Find either an amp or speaker you really want, then match it with what you need.
Find either an amp or speaker you really want, then match it with what you need.