Finding (calculating?) speaker and amp impedance

I don't know how to do those calculations but I can say I have never been unable to find that information by just using Google or an email to the manufacturer.  Not the answer you were looking got but maybe helpful?

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.

Wow! Lots to consider.

I guess my real concern is knowing if my amps have "high enough" current for speakers that present a challenging load.

Surprisingly (to me) was finding out that my old 80 db sensitive little monitors are known to be great with tube amps. These are Rogers LS3/5a from the early 70s.

@erik_squires is there anything you know of that will measure speaker impedance across the frequency range?

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