Finding (calculating?) speaker and amp impedance

@nonoise True, but you also have to then accept the ADC / DAC of the amp. If you want to keep an analog chain after that DAC you spent 2 years pickinmg out this tech is wrong for you. :)

Technics does and employ something like their LAPC function which "looks" as the speaker impedances with a series of test tones and adjusts the amps output to match it as closely as possible.

@nonoise

I must admit that this is a pretty cool application, but kind of limited in benefit. Perhaps the biggest benefit is to switching amps which tend to suffer (relatively) high output impedances at high frequencies.

@asctim  Outside of exotic manufactured speakers like full-range ribbons or ESLs, I think there are a couple of reasons.

One is lack of decent tools and practice.  Old Genesis speakers sometimes showcase this.  The crossovers were "tuned" by ear and impedance be damned.  Given that a software spreadsheet had just been invented, there was not a lot of handy tools like we have now to tune a speaker's crossover for impedance, frequency response simultaneously. 

Another, and I've seen Focal do this, is that a difficult to drive speaker is seen as "discerning."  No, I'm not going over this detail, either you believe me or you do not, but reviewers give far too much positive press to a speaker that shows you the difference between an integrated and million dollar monoblock. 

Lastly, and I think the Kef is an example, is that they have a big problem with power efficiency and need a lot more current to make the design work.

Forgot to mention, you CAN measure this yourself using Dayton Audio DATS or a rig built for Room EQ Wizard.

If not published by the vendor/manufacturer, is there a way to determine the impedance curve from the specs that are provided?

Sadly unless some one like Stereophile or Soundstage reviews and measures them there is not. At best many of these measurements are "aspirational."  There's no math, it's all eyeball and feelings between the complex curve and published nominal value, so vendors have a lot of leeway and there's no way to reverse engineer from the rated impedance alone.

A vendor appealing to tube amp lovers may specify both a "nominal" (i.e. in name only) measurement as well as a minimum impedance which is really useful. A typical minimum for 2 way speakers for instance is around 3.8 Ohms.

Recently in another thread we discussed a Kef monitor which is nominally 4 Ohms. In reality is is a 2 Ohm speaker, so this is how far the gap can be.

One thing you may do is seek out speakers with series crossovers like Fritz which have very flat impedance curves compared to other speakers. That tends to make them easier to drive and provide more consistent performance across different amps.

I mean if you look at a lot of Stereophile measurements you kind of get a sense for how a typical 2 or 3 way speaker will behave, but then there’s no guarantee at all that say a monitor like the Kef is going to be at all typical. In addition to driver impedance a lot of equalization and level adjustments happen in the crossover. That can make some impedance curves look very different from another.

Forgot to mention, you CAN measure this yourself using Dayton Audio DATS or a rig built for Room EQ Wizard.