Why are low impedance speakers harder to drive than high impedance speakers

Also remember the complexity of the Crossover is directly proportional 
To ohms,resistance to the drivers and demand affecting  the Amplifiers 
Constant changing ohms ,and Amplifier Load depending on demand. 
A speaker can  go from a 8ohm load to 2 ohms which Is very demanding 
For a conventional driver Loudspeaker. Panel or stats ,can dip down to 1ohm .
This  is where a Amplifier be rock solid .verify that the  amplifier you are using 
Can handle the extreme for a given Loudspeaker before purchasing it.

Hi Bdp24,
For sake of this discussion I’m referring to speaker load impedance characteristics that remain "relatively flat. So my hypothetical comparison would be a 4 ohm vs a 16 ohm. A speaker such as the Quad that you cite with its extreme fluctuations renders nominal rating meaningless lf both the 4 and 16 ohm speakers are stable, the higher ohm speaker demands less current and work from the driving amplifier as predicted by Oohm’s law.

In reality there are speakers with both reasonably flat impedance characteristics and gentle phase angles. There are also those with widely fluctuating impedance and steep phase angles. It seems that the former would be preferable as it is far less demanding on a power amplifier assuming that either provides comparable sound quality. 
Charles

Hi Charles,

Usually it comes down to approach: picking the speaker you love and then finding the right amp or vice versa, or in a fortuitous alignment of skill and circumstance, finding both without significant compromise.

Best to you Charles,
Dave

bdp24 1-8-2017
For instance, the original Quad ESL's nominal impedance was 16 ohms, but it’s impedance rose to 60 ohms at low frequencies, and fell to 1.8 ohms at high frequencies---anything but an easy load! That impedance characteristic is one reason the sound of the Quad ESL is so affected by the amp driving it, and why almost no solid state amp is a good match---it makes for overblown bass and missing highs.

BDP, a minor correction to your characteristically excellent inputs.  The last phrase should be "it makes for missing bass and overblown highs."  As you no doubt realize, and aside from some rare exceptions, in comparison with a tube amp a solid state amp will deliver more power into low impedances and less power into high impedances, for a given input signal level.

Best regards,
-- Al
 

 ^ Lest anyone get the idea that the increase/decrease of power output by ss amps into decreased/increased impedances would suggest that frequency linearity would be compromised, the opposite is actually the case, they actually provide better frequency linearity. Most typical speakers will decrease/increase their sensitivity in direct proportion to the increased/decreased impedance changes.
 Furthermore, there are many other considerations that speaker designers need to consider besides impedance. With the advent of amplification that can adjust power output to speaker impedance/sensitivity, speakers designers were now able to permit more attention on other speaker considerations such as wave form fidelity, dispersion characteristics, box resonances, etc..