Speaker impedance patterns - how to read them?

So here you go:
Most conventional cone speakers have a rising impedance as frequency goes up. What most speaker designers should do that do not want to impedance compensate their designs is rate the impedance based upon the operating range of the individual drivers. Most companies do fudge numbers, if a speaker that they are currently calling an 8 ohm speaker showed that dipped into 3 or 4 ohms, many tube aficionado’s would not buy them. A realistic example would be: if you have a 2 way and the woofer is rated at 8 ohms but actually varies from 4 at dc to 30 ohms at 4000 hz. Realistically that woofer will range from say 6 ohms at its 3db down point of say 35 hz or so to a 15 ohms at a crossover point of 2k.... So in the woofers operating range, it varies from 6 to 15 ohms. it is called an 8 ohm woofer.
Rising impedance can easily be controlled. So in the example above, that 30 ohms could be controlled to stay within 6 to 8 ohms. My last design is a 15 inch 2 way, it is 95 db in sensitivity and varies from 5 to 6 ohms at all frequencies and yes, even though a solid state amplifier handles rising impedance easier, it is still better for an amplifier to see a very consistent load and if your tubed amplifier had 4 ohm taps, it would much prefer a impedance compensated speaker. Normally overall, phase angles are tube amp killers as much as just impedance, a tubed amp lover should look at phase angles as well as impedance curves.
There are the basics with a very basic example. There are times that other scenario’s could happen, but the above is the most common for conventional drivers. I hope this helps, Tim

Most conventional cone speakers have a rising impedance as frequency goes up.

I’m sorry that’s not really true. You are describing the impedance of a coil.

It is true that larger cone drivers have this, and that this effect needs to be dealt with but it is not true for a speaker as a whole. A quick survey of impedance curves at Stereophile would show this.

Here’s a random sample:

https://www.stereophile.com/content/stenheim-alumine-three-loudspeaker-measurements

It is also true that the most challenging areas for a conventional speaker tends to be in the bass, but the impedance curve of a raw woofer is not at all a good indicator of the impedance curve of the speaker as a whole.

You are starting at the wrong link in the chain  start with speakers you like what if you don't like any speakers with your amp and you can crunch all the numbers you want its not going tell you how they sound demoing is the only way if your dealer don't have your amps bring the amps there 

Enjoy the Music
Tom

Actually Eric,  it is absolutely true.  You provided one driver.  I've measured hundreds of impedance curves not a couple.  
Yes,  I used a woofer as an example, but this happens in many mids, tweeters etc and their individual impedance curves do show up on a graph.  Of course, I've stated often on this site, overall, I do prefer and recommend impedance compensation networks. 
Unless you are describing some sort of ribbon/amt's which tend to have a very flat impedance curve. 

Basically the frequency response of an amp with a low damping factor(high output impedance) will be affected by the peaks and dips of the speaker. The amp even if it's flat into a test resistor will not be flat into a reactive speaker load. The amp response will have dips and peaks at the same places as the speaker impedance curve. How big these will be depends on the damping factor(which can vary with frequency by the way). The higher the damping factor the smaller the changes in response until the damping factor is high enough so the amplifier frequency response is flat with the speaker.

I recall a letter years ago to Hi Fi News that said that using very low resistance speaker cables minimized the affect for low damping factor amps. Frequency response was shown for normal resistance cables which exhibited the frequency variation. But with very thick cables(about 4 gauge if I recall correctly) the frequency variation essentially was gone. But this was before modern cable technology and just changing to very low gauge cables will probably also affect other variables in cables which may or may not be good.