The two most common mistakes are bass and treble

Agree with the requirement to try them at home first. Many showrooms have odd acoustics and it’s not always possible to properly place the speakers in that room. 

Not saying you are incorrect, but give me an idea of a couple of common inefficient speakers, and a couple you think do a great job.

What is your cut-off for efficiency and impedance, more or less? What is good, and what is bad?

What a joke about not purchasing speakers with sensitivity ratings lower than 90 or lower than an 8 ohm load. In this 1 sentence, you pretty much eliminated every "best" speaker on the market. Most of these "best" speakers have sensitivity ratings in the middle to upper 80's and some go down below 4 ohms (some in the 2 ohm range). All this means, if you buy the best speaker, you also need to buy a better/best amp to drive them.

A speaker that is low efficiency (less than 88db if 8 ohms) is simply not 'the best' plain and simple. It might be very good, but not the best. Its a common misconception that when the speaker is inefficient it trades off efficiency for transparency. This idea is simply false. One of the most transparent speakers I've heard (and this is compared to Wilson, Magico and others) is 98dB (and also is flat to 20Hz), made by Classic Audio Loudspeakers. Transparency and efficiency are unrelated.

IMO/IME when the speaker gets below 89 dB you are into a situation known in the radio world as 'gold plated decibels'. You also have to pay attention to impedance; if the speaker is 89dB and 4 ohms its actual efficiency is 86dB, because 2.83 volts into 4 ohms is 2 watts, not 1. Think about it this way: in most average rooms to work with a speaker that is 89dB and 8 ohms you'll need about 250 watts to really play whatever you want without clipping the amp. But if the speaker is 95dB you'll only need 60 watts to do the same thing. Its far easier to find a good sounding 60 watt amp than it is to find one that makes 250 watts. As the power goes up, the field narrows dramatically!


Amplifiers might be able to drive 4 ohms and less, and they might be able to double power well below that, but because they can do that isn't the same as saying that amplifier is sounding its best. This is all about distortion: its distortion that causes us to hear differences between amplifiers. The brightness coloration of solid state is caused by distortion- the higher ordered harmonics, even though the THD might be only 0.01%. The ear uses the higher ordered harmonics to sense sound pressure and so is keenly sensitive to these harmonics.


Its a simple fact that all amplifiers have less distortion into higher impedance. You can see it in their specs and you can hear it as well. If you want a solid state amp that suffers from brightness to sound a bit smoother and more detailed, have it drive a higher impedance.


Not all solid state amps have brightness- this coloration occurs when the amp had insufficient feedback which is describing most of the amps built in the last 70 years. You can get away from this by having no feedback at all, or you can get around it by having really a lot. But its important to understand that you can't just go out and 'buy a better amp'; there are not that many really powerful amps that sound like music.


The other problem you deal with is of course that you get thermal compression if the speaker lacks efficiency. You can't solve this by getting a more powerful amp, it gets worse the more power you put into the speaker.

P05129 that’s what I mean- so many people try to speak with authority and absolutes but they don’t know jack other than what they know and like.

I apologize if I sound like this- I'm coming from the perspective of engineering and as an amplifier manufacturer of the last 45 years.

Hello,
I feel so fortunate that the store where I get my Hifi gear lets you try before you buy. A few of my pieces of gear would not have ended up in my system like the Puritan PSM156 or the Nordost digital coax cable. I am excited because they have become the Ayre dealer in the Chicagoland area. Demoing gear in the home is the most important thing you can do. Even if you bring in your gear to the store it will not sound the same. If you are in the Chicagoland area and want to demo things right try: https://holmaudio.com/

I’m coming from the perspective of engineering and as an amplifier manufacturer of the last 45 years

Within a specific power envelope ?

The other problem you deal with is of course that you get thermal compression if the speaker lacks efficiency. You can’t solve this by getting a more powerful amp

No, you can solve it by various aspects of speaker driver and crossover designi. I don‘t think anyone could state that the ATC SCM50 (85db/W/m) suffers unduly from thermal compression. There are many other manufacturers who are cognisant of the impacts of thermal compression and design their drive units accordingly.