As many have posted, the nominal impedance varies with frequency. Getting lower efficiency speakers to make a sound with a few watts isn't that difficult, but getting the most out of them with insufficient power is. Nearly all sensitivity ratings are made at 1000 Hz, so it's just a tiny slice of the spectrum we're attempting to amplify. For example, if the speaker is 3 db less sensitive at 100 Hz, it will take 2x as much power at that frequency to produce the same volume as the 1000 Hz tone. And if the impedance drops to 4 ohms, for example, at 100 Hz, then the current demands will be higher for 100 Hz than 1000 Hz. I used to have Thiel CS3.5. They were the first speaker I owned that really needed power. They were excellent speakers for weeding out the amp collection. Then I discovered Magnepans, which had a hard to drive, must have power reputation. Yet they weren't nearly as difficult as the Thiels. None of my tube amps could drive the Thiels well, while the Maggies love tubes. Moral of the story, don't read too much into the specs. They're a single data point. Music is a nearly infinite amalgam of data points when you consider the frequencies and amplitudes at play. Listen to speaker candidates with the amp(s) you have, or better yet, choose the speakers you like best, then find the amp that pairs the best with them.
Basic question about sensitivity
Obviously I am mistaken, and I am positive that this has been covered before, but I had thought that the higher the number (sensitivity) the easier the speaker was to drive. However, when I read a thread in which someone is talking about a speaker they have or are interested in, I frequently do a cursory search and a little bit of reading about it, and, for example, a while ago I read a review & specs on the Monitor Audio Gold 300 5G with a sensitivity rating of either 90 or 91 (which I would have thought at one time meant it was a pretty easy load to drive) but the recommended amp was 100 to 250 wpc.
On the other hand, I just did a search and some reading on the Harbeth P3ESR XD which has a sensitivity rating of 83 (which at one time I would have thought meant it was a tough load to drive) but they are recommending amps "from 15 wpc".
What is the number listed for a speakers sensitivity actually meaning?