More resistance is less load??

OK- now we got the basic part- a greater load is less resistance, less of a load is higher.

There is more of a take-away than this however. It has to do with power vs distortion (and also in the case of tubes bandwidth may be affected).

**If** high fidelity is your goal, your amplifier dollar investment will be better served by a loudspeaker of higher impedance (all other things being equal). This is because, regardless of the amplifier technology (tube, traditional solid state or class D), the distortion will be higher driving lower impedances. This is both measurable and audible.

**If** your goal is sound pressure, there is a slight argument for a lower impedance speaker (in the case of an 8 ohm speaker, a 3 db higher sound pressure will be had if you go to 4 ohms) all other things being equal. This however is only true if you have solid state, and still might not be true unless the amplifier can support the additional current required to double its power (a 3db increase in volume requires a doubling of power, and cutting the load impedance in half could win you that 3 db).

The kind of distortion that is increased in all cases by the use of a lower impedance load is audible, despite the increase usually being rather slight. This is because the additional distortion produced is of the kind to which the ear is extremely sensitive: higher ordered harmonics and additional IM distortion.

The presence of distortion obscures low level detail due to the ear’s masking principle. The ear/brain system converts all forms of distortion into tonality; thus the slight additional distortion contributes to brightness and harshness.

As a result, generally speaking, any amplifier will therefore be smoother and more detailed driving a higher impedance load. Its a simple fact that you can make an amplifier work hard by making it drive a difficult load, but that it not the same as having it sound its best!

I know that many people have fallen in love with certain low impedance speakers (many of which have excellent properties) and they are also quite happy with the combination of amp and speaker that they have. All I am pointing out is that if that same speaker were somehow 8 (or better yet 16 ohms) instead of 4 ohms, it would sound smoother and more detailed with no real downside except a slight amount of power in the case of solid state amps (tube amps would make the same or slightly higher power, likely with wider bandwidth as the output transformer is more efficient driving higher impedances).

For speaker designers, a simple way to make their speaker seem smoother and more detailed is to simply make it higher impedance...

^^ Yes. Take a look at the specs and you will see.

there might be measurements that suggest that there were other benefits that made foregoing better measurements at the amplifier worth it.

That measurement is power, which equates to sound pressure. Otherwise there is no real advantage to a low impedance load.

One thing I forgot to mention is how much more important the speaker cable becomes when driving a lower impedance. Into 4 ohms the speaker cable is critical and differences are easily heard between them as a result. Conversely, at 16 ohms the speaker cable is far less important and longer runs can be made with less deleterious effects.

Into 4 ohms, the speaker cable can easily degrade the damping performance of the amplifier. So if your speaker requires a higher damping factor (+20:1) then it will be important to keep the cable as short as possible to minimize its effects. I've seen this have dramatic effects on the resulting bass impact!

It's easier for a speaker manufacturer to get a steady impedance (which amongst other things increases the potential for frequency linearity) with lower impedances than higher numerical impedances.

This statement is false in several ways. Impedance has nothing to do with the linearity of the impedance curve- that has entirely to do with the design of the speaker. Secondly, a linear impedance curve likely benefits a tube design rather than a solid state design- the real issue is whether the amplifier can operate as a true voltage source- if so the linearity of the impedance may well be moot.

A long time ago amplifiers had a great deal of trouble dealing with low impedances and high power. With the advent of reliable ss amplification that can double down, speaker designers were free from those constraints to advance other aspects of speaker design.

This statement is also false. What really happened in history is that the idea of an amplifier being able to operate as a voltage source was proposed by EV and MacIntosh in the 1950s. The idea was to eliminate the guesswork of setting up a loudspeaker. At the time, a speaker was usually set up with a midrange control and a tweeter level control. These were not there to adjust the speaker to the room, they were allow one to adjust the speaker to the voltage response of the amplifier used (as such, both the speakers and the amplifiers in use would be what I call Power Paradigm technology).

With the introduction of the Voltage Paradigm, level controls on the speaker were no longer needed.

At the same time, the industry was transitioning from tube power (which is expensive, so much so that most loudspeakers were fairly high efficiency) to solid state. It was a lot cheaper to build a solid state amp (no filament circuit and no output transformers) and yet the industry was able to charge nearly the same money for the amps. Solid state amps, requiring a lot of feedback for linearity, had a side benefit from that feedback (and the otherwise lower output impedance of the output devices) which was that they could much more easily operate as a voltage source.

The significance of this (especially the increased profit) was not lost on the loudspeaker industry. It takes a lot of precision (costs more) to build high efficiency loudspeakers, and so with the new higher powered solid state amps, it was possible to build lower efficiency speakers and yet **charge nearly as much for them** (and if you are following the dollars here then you see what this transition was really all about). To get back some of the loss in efficiency, loudspeakers began to appear that were 4 ohms rather than 8 or 16 (or even 32 ohms...). The lower impedance asked more power of the solid state amps, and so a speaker might be 10x less expensive to build and not really seem to be all that much harder to drive (while costing the customer nearly the same retail dollars).  Of course the amplifier ran hotter...

In time solid state amps also became reliable driving these impedances, but to say that there was a constraint removed to advance speaker design is not at all accurate, unless you look at it from a perspective of profit.

Yes- you get lower power into higher impedances with solid state amps. But you also get lower distortion- essentially the amp will sound smoother and more detailed as a result.