Low Amplifier Loads / frequency range

It depends on the speaker design. see:

http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php

If you are designing for tubes, keep the load easy (or 16 ohms) and you will do very well with no need to biamplify.

Its a huge myth that you need large transistor amps to 'control' the woofers! The truth is that if you design the woofer array for the job, even dual 18" woofers can be handled by a small tube amp quite easily.

Thank you much. I read the reference posted on your
website. It seems to make sense, but I don't have an
Electrical Engineering background and cannot digest
all of it.

It leads me think that I may not be asking the right
question, I'm not sure.

In my 4-way speakers, there will one amplifier directly
connected to one driver. So won't the load that each amp
sees depend only on the signal frequency going to that
individual driver ?

I can tell you only the woofer will be operating near
or at resonance conditions. The other drivers have been
chosen such that they will not be playing near their Fs,
therefore no signal frequency fed to them will cause
resonance (I think).

Without resonance, the 3 upper drivers will not produce
any unduly low impedances for their amps, within the band
of frequency each driver is fed (I think).

Am I OK this far, or am I already off ?

Because if I'm correct, only the woofer amplifier could see
exceptionally low impedance. If I'm wrong, then I must
learn.

Generally speaking the impedance of a driver is dependant on the type of driver and manufacturer.

Large bass drivers often have an impedance dip at the lower end of their frequency response. Whatever amplifier you use, it must be able to drive the chosen driver at it's minumum impedance. The power needed depends on the efficiency of the driver and to some extent the cabinet configuration the driver is mounted in.

Mid-range and tweeter drivers may also have a low impedance (compared to 8 ohms) but are often more efficient and need less amplifier power to produce a given SPL. So generally speaking, the amps driving these speakers do not have to have the same power levels as the bass amp. That said, you may still want very fast (high slew rate) amplifiers for the mids and tweeters, and often high slew rates go hand in hand with high current output capability.