I'll try to be that "guy in the corner nobody talks to" and shed a little light on the topics raised by the OP.
Impedance is somewhat like resistance conceptually, but for AC (as in Alternating Current like the power delivered to our homes or a music signal). In fact, resistance is a component of impedance which also consists of inductive and capacitive reactance. What's confusing is that impedance is often measured in ohms, like resistance. It is helpful to see impedance denoted by a "Z" to differentiate.
Another notable characteristic of impedance is that it varies with AC frequency. Speakers can have a "nominal" impedance, such as 8 ohms, but that is an average value. Speaker impedance varies greatly as you can see in the test results published in Stereophile, by manufacturers, and other places. The impedance curve is a result of the drivers, cabinet configuration, and crossover. Speaker impedance generally dips to half of the nominal value, so a nominal 8 ohm speaker will go as low as 4 ohms, and a nominal 4 ohm speaker will goes as low as 2 ohms at some frequencies.
Amplifiers also have an output impedance. Some amplifiers have a very low output impedance of just fractions of an ohm. These low-Z amps will often use a follower design or employ generous feedback. Low-Z amps are often solid state, but tube amps can be built with low-Z as well. Other amplifiers use very little, or no, feedback and can have an output impedance of several ohms. High-Z amps are often tubed and can be SE and PP. However, there are a few high-Z solid state amps such as some of Nelson Pass' designs.
Where it gets interesting is how an amplifier interacts with the speaker it is driving. The ratio of speaker impedance to amplifier impedance is known as the "damping factor." A high-Z amp such as no-feedback SE 300b with an output impedance of 2 ohms has a damping factor of 4 when connected to a nominal 8 ohm speaker. At the other extreme, Benchmark publishes a damping factor of 254 into 8 ohms for the AHB2 at 1Khz. That calculates to a very low-Z out of ~.03 ohms.
What this means is that a low-Z amp will output the same voltage regardless of the frequency and speaker impedance. Whereas a high-Z amp will "sing along" with the speaker as the frequency and impedance varies. Some speakers are design to work with a high-Z amplifier. Most others are not.
Here are a few things that can contribute to a speaker being considered an "easy load":
- High nominal impedance (results in a higher damping factor)
- "Smooth" or flat impedance across frequencies (minimizes voltage variance with a high-Z amp)
- High sensitivity (speakers can play loudly with little power from an amp)
The OP described Revel documentation warning against using speaker cable with more than 0.07 resistance. This implies to me that the speaker was designed to be driven from a very low-Z amp and that even a small amount of output (or cable) impedance will affect the performance of the crossover and sound.
I hope this is helpful?
