Multiple speakers with Sonos Amp

That is not what happens in a speaker, the impedance varies due to the capacitance, reactantance and resistance of the crossover network at various frequencies, not because the speaker needs more energy. It is a very complex relationship as the amplifier is usually feeding a variety of frequencies to the crossover and the impedance may be very low at one frequency while being much higher at a different one.

Speakers do draw power to produce sound - power isn't fed into the speakers in the absence of such a draw. For instance, when the speakers are silent, there is very little power being drawn by them even though the amp may be on and volume may be on full. The amp, using the common water analogy, is a reservoir of power, not a pusher of it. Of course, the amp, depending on what class it is, will have quiescent power consumption all the time it is on, but this will not be a large amount unless the amp is of the class A type.
And speaker impedance - resistance across the speaker terminals - does drop precisely because the speakers need more energy and the lower resistance allows for more to be sucked out of the reservoir - akin to increasing the size of the pipe.

So my thread has now become very theoretical. It’s interesting but hard to take practical guidance from. I’ve been recommended four Dali E80s (6ohm) with a Sonos amp. Is this a good combination, do I need an impedance matching unit and if so which sort? Or should I just choose 8 ohm speakers instead to avoid the inefficiencies of an impedance unit?

I surrender.

4 6 Ohm speakers are going to be roughly 3 Ohms, I wouldn't bet on them working particularly at higher volume levels. Why not contact Sonos support for an official answer? The twitter option is open 24x7 and the phones will be open Monday morning. Either option beats getting an unusable combo.

There is no dearth of very good 8 ohm speakers out there, so there isn't any need to be different except where one is looking at legacy/gifted units.

Ultimately, an amplifier is rated for a certain amount of output current before it burns up. An amplifier is really a constant voltage device. As you add more speakers the voltage output remains the same (for a given input voltage) and the individual speakers are not aware of the others. The output level of a given speaker does not change as additional speakers are added. This continues, as more speakers are added, until the current limit is reached. Beyond this point the amplifier takes protective action or burns.

It’s a similar situation for a lamp circuit, but we use different math. We keep adding lamps until the circuit limit is reached. Beyond this point a fuse will blow or the house will burn. Mercifly, we specify a lamp’s power draw at the rated voltage, rather than the lamp’s resistance.

The math for consumer audio speakers is contorted (for reasons that we don’t need to get into here). In professional audio the amplifier’s output voltage is specified along with a speaker’s power rating — just as our lamp circuit. One keeps adding speakers until the amplifier power rating is reached. It’s the same physics, just simpler math.

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An impedance matching device alters the voltage to current ratio for the circuit. If the current (and output level) is reduced for a given speaker, more speakers can be added to the circuit.