The impedance of a speaker is a very slippery concept. If you plot the impedance of a speaker (particularly a woofer) vs. frequency, it looks like a mountain range. There is no consistency whatsoever. Part of that is due to the enclosure and the effect it has on the speaker. Bass reflex? Acoustic suspension? Horn? Yes, there are a few horn woofers. So you look at the graph, squint, and pull a number out of midair and say that it's an 8 ohm speaker. This has little to do with the DC resistance of the coil.
The amplifier doesn't know what the speaker impedance is, it just drives as hard as it can and hopes for the best. It's designed to drive almost any speaker. The limitation is that as the impedance drops, the amp has to put out more current. At some point, it runs out of room and the output clips or does some other ugly thing. Maybe a fuse blows. In the days of tube amplifiers, they often had different output terminals for different speaker impedances. This helped balance the output to the speaker which was more important for tubes. Tube amps can't put out as much power as transistor amps can, so the speakers had to be more efficient.
The biggest thing that affects speaker efficiency is the enclosure. A horn is the most efficient, followed by bass reflex, then acoustic suspension. If a horn needs one watt to reach a particular loudness, an acoustic suspension might need 20 or 50 watts for the same loudness. That's why large outdoor venues often use horns. They need lots of sound! In your home, efficiency doesn't matter. The frequency response, dispersion, even the look of the thing, are more important.
Ed