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

I was surprised when I learned that some choose their power amp first, then look for a speaker it can drive well. I thought the notion that speakers, being transducers, vary much more in sound that do amps was universally agreed upon, and should therefore be selected first. The notion that power amps vary in character as much or more than do speakers is one I disagree with. I feel the same way about phono cartridges (also transducers) vs. pickup arms and/or turntables, though to a lesser degree.

Excellent clarification from Al, with more specifics. No surprise there! Timbre77, you missed the important qualifying adjective of the amplifier scenario I described---"tube". As Al stated, while the power solid state amps create increases with dropping impedance, the opposite is generally true of tube amps, with the notable exception of the unique Music Reference RM-200, which actually behaves more like a ss amp in that regard.

As Al detailed, the different impedance taps on a tube amp allow the amp to provide similar power to all the taps---that's one task of tube amps output transformers. However, if your tube amp has 4, 8, and 16 ohm taps (typical in tube amps, though the RM-200 offers 2, 4, and 8 ohms), and you hook up an, say, 8 ohm speaker to the 4 ohm tap, the power available to the speaker will be less than it would be if connected to the 8 ohm tap. But, says Music References Roger Modjeski, a tube amp so employed will usually be producing not only less power, but also less distortion, and better sound. As Al mentioned, Roger calls this tactic "light loading". In addition to lower distortion, an additional benefit of using a lower impedance tap is that the amps output impedance will be lower---it will have a higher damping factor, and will interact less with the varying impedance characteristics of the speaker load, resulting in a more predictable frequency response.

While a 16 ohm speaker may seem to "present an enormously easy load for virtually any power amplifier", such is not necessarily the case, as that 16 ohm rating is merely nominal. 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. Ralph Karsten of Atma-Sphere has already explained this a few times.

Related to my post above, using the lowest impedance tap on a tube power amp not only usually results in the lowest distortion and best sound the amp is capable of, but the amp’s damping factor is also highest, leading to the flattest frequency response it is capable of, irrespective of speaker impedance characteristics.