No, you’re wrong with this statement because impedance by definition is not a constant thing, it changes significantly with frequency. Partly an even impedance represents the speakers that are easy to drive but that’s not everything too.
Efficiency has nothing to do with ease of drive from the perspective of the amp. It simply means that for any level of electrical input the sound produced will be louder. The amp only ‘sees’ the (reactive) electrical load, not the efficiency.
I said impedance curve before.
It means the amplifier will need to produce more power to drive the speakers to the desired level, which is not unlimited, hence a more difficult load.
Providing more power within the amplifiers limits, is not a difficult thing for an amplifier to do! It is just more electricity used!
Although clearly you have a different understanding of what it means to be for a speaker to be difficult to drive, as I pointed out before, in hi-fi when people talk about speakers being difficult or easy to drive they are not normally talking about how much power is needed to reach any given volume level, nor whether the amplifier will be working nearer it’s maximum power capability to play at whatever volume they want, but whether the amplifier is able to properly control the speakers. In that respect a key factor is how varied the impedance curve is, in particular whether it has any very low points within the audio frequency spectrum even though the average may yet be its nominal 8 ohms or 4 ohms or whatever. Factors affecting the impedance and demand on the amp include crossover design and cabinet design as well as driver parameters.
That is actually called Damping Factor.
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“damping factor”, the speaker impedance to supply impedance ratio, indeed is a factor in controlling the cone, the back emf arising from movement after power ceases when effectively “shorted” by a very low impedance across the amp output terminals via the speaker cabling effectively acting ax a brake. But that is only one part of bass control: the cone must be driven effectively when power is applied, moving as near instantly as possible, and moving the distance required before return. If at a particular part of the frequency spectrum the impedance dips deeply, a lot more current must be supplied compared to another frequency. If the amp can’t supply adequate current, and fast enough, the cone won’t move as it should so the range of frequencies affected by the impedance dip.will not reproduce correctly - it takes a more capable amp to respond correctly. This phenomenom almost invariably happens in the bass region.
There is theory, and there is reality.
I have heard many speakers with high sensitivity over years, or read reviews, be it Avantgarde, Living Voice, De Vore, Tannoy, Audio Note, Klipsh,….all having more than 98 db . They were all driven by low power tubes amps, between 8 watts to 20 watts.
All were easy to drive.
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Has anyone said otherwise of the speakers you’re referencing? Many horns are simple designs electrically, while they have acoustic loading that is approaching perfect. Also, whilst I don’t know about the particular models you’ve heard, a lot of horn speakers don’t really do bass. In my case the one point I have been making here, in response to a statement suggesting otherwise, is that a speaker’s efficiency is not an indication of how easy or difficult a load the speaker is, in terms of what people in hifi normally mean when describing a speaker as easy or difficult to drive.
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To get back on topic, I think the P3ESR XD are an exceptionally musical pairing with the SN3. Specifically this model (I’ve heard the larger ones as well) but it will need a sub. Harbeth do sell a recently developed stand that contains a sub for these.
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Maximum power and continuous power ratings are essential factors contributing to an ideal sound system. Thus, it’s important to know what these two values refer to when purchasing speakers, amplifiers, or subwoofers.
These days, most amplifiers come with a list of compatible ohms, making it easier to match them with loudspeakers without worrying about impedance.
Indeed not; efficiency has nothing to do with ease of drive as Higher efficiency means louder sound: An efficient speaker can produce a higher sound output with the same amount of power input compared to a less efficient speaker.
Efficiency and sensitivity go hand in hand: Speaker sensitivity, which measures how loud a speaker is at a given power input, is directly related to speaker efficiency
Speaker sensitivity measures how efficiently a speaker converts electrical power into acoustic energy. It is often expressed as the sound pressure level (SPL) that a speaker produces at a distance of one meter when driven by one watt of power.
Efficiency, and Speaker Impedance: Speaker impedance, measured in ohms, is the speaker’s electrical resistance to the amplifier. It plays a significant role in determining the sensitivity and efficiency of a speaker.
The speaker enclosure’s design and the speaker’s placement in your audio system can also impact its sensitivity and efficiency. For example, a well-designed enclosure can enhance the sensitivity and efficiency of a speaker by minimizing resonances, reflections, and other distortions that can affect the overall sound quality.
Similarly, correctly placing the speaker in the room, considering factors such as distance from walls, height, and orientation, can also affect its sensitivity and efficiency. It is essential to consider the recommended enclosure design and placement guidelines provided by the speaker manufacturer for optimal performance.
anyhow, about the OP’s question…
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And there are facts. 
My Klipsch Heresy III were stated to be 99dB (actually about 96). They were also specified for 100W minimum, max 400. I wrote to Klipsch themselves to solve my doubts re: the rumours about very low power, tube amps amps being ideal for them, the reality of the specs and the complex crossover. A very kind member of Klipsch said to me that the late Paul Klipsch used solid state cinema power amps in the hundreds of watts power range.
My Heresy IIIs’ large woofer had stiff suspension being designed for a sealed box and a short excursion of the voice coil; it was undoubtedly easier to control than a more modern design with softer surrounding but needed watts.
On the other hand, LS3/5As are universally considered (by pro-reviewers) to be extremely tube friendly in spite of the 82.5 dB sensitivity because of a benign impedance and phase module. When Paul Klipsch said his ubiquitously quoted thing about a good 5-watts amp being all that was needed by the world, his Heresy Is had a super simple crossover and their max power handling was about 50 watts, perhaps less.
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Is it a problem to have a few audio myths clarified for the benefit of all?
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That sounds rather twee, I guess AI generated? Apologies if it is your own wording.
Efficiency and sensitivity are often conflated but aren’t the same thing at all. In nearly every instance where published specifications quote “efficiency” they actually mean “sensitivity”. I can hardly think of any manufacturer that published true efficiency, mostly because they are all hovering around 3% and it isn’t a helpful number.
As to sensitivity, it’s perhaps best to just use it as a starting point for drivability but not the answer. Some publish this as an average of frequencies, and many publish sensitivy as just 1w @ 1Khz. So you can easily end up comparing apple and oranges.
Assuming the measurement is consistent, at best it tells you how an amp may be able to fill a room to a realistic listening volume. A 10w amp is never going to fill a tennis court sized room driving 84db speakers but they might be fine together in a converted shed.
As has been stated further up, phase angles and impedance curves are what really determine driving difficulty. This is where transient current delivery is needed more than constant power output. Something Naim amps excel at. So use sensitivity as a starting point, yes. But bear in mind there are 84db tube amp friendly speakers out there and 102db speakers that are a real devil to drive.
A knowledgeable dealer can help here. Their experience with driving different speakers they stock will be more valuable than the spec sheet sensitivity (or erroneously labelled “efficiency”) value.
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Totally agree…ease of drive and efficiency are definitely not related…
There’s theory, there’s facts, and personal tastes.
I don’t deny that some high sensitivity speakers work better with solid states or relatively high power amps. But it’s rare however.
As for LS3/5A , i have never heard.
I heard the Klipsh RF7 with an Audio Analogue solid state integrated, with 100 W, and the sound was nice.
With the Prima Luna tubes, it was lifeless.
Electrical damping is determined by the resistance load on the loudspeaker, which is the sum of the loudspeaker cable resistance and the amplifier output impedance.