Finding (calculating?) speaker and amp impedance

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.

It is a measured parameter. The speaker designer needs to have this as a priority in their design. One speaker designer who does a great job with flat impedance/response and phase angle is Eric Alexander.

I was just thinking about this issue this morning. Those who design speakers that present very difficult loads to amplifiers must believe that there is no other way to get as good a sound, otherwise why do that? 

@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.