Most speakers today fall in the range of 87db to 92db. Keep in mind that this specification is an average across the frequency range, and amplifier demands vary even between speakers rated at the same efficiency levels. Lastly, these specs have absolutely nothing to do with sound quality. Active speakers have power, as do active subwoofers. Active means that the speaker or subwoofer has an amplifier built in.
Most subwoofers are of the active variety, whereas most speakers are of the passive type.
A common trend recently has been to combine these, into speakers with built in active subwoofers. This makes a lot of sense, because bass woofers take the most power, but your existing amp is only running the midrange and tweeters, which are easy to drive. This is the specification that really counts, but you will not find a number or formula for it. Many components with identical specifications still sound noticeably different from each other. Human nature as it is, we strive to define and categorize the sound of components.