Disclaimer: I'm an electrician, so this explanation may not be 100% correct. Electronics and tubes are weird.
- A better rule to work with, as he mentions in the article, is "amp and cab can be one 'step' different from each other" - i.e. an 8 ohm amp and a 16 ohm cab, or a 4 ohm cab. Both situations will still have the negative effects he discusses, but they'll be fairly mild.
- Output Impedance can be thought of as how hard the amp is expecting to work; each setting is just taking a specific voltage from the output transformer, which provides an acceptable/safe amount of current when you give it the appropriate speaker.
- If you hook a 4 ohm speaker up to an 8 ohm output, the amp is still providing enough power for an 8 ohm load, so you're pumping twice as much current through both your power tubes and the speaker. Current makes heat, heat wears things out.
- Likewise, if you put a 16 ohm speaker on an 8 ohm output, and don't change the volume, the speaker is only seeing half as much current and your volume will drop. That's not so bad by itself, but because a speaker is an inductor (read: a coil of wire) it also pushes some current back toward the amp. This, in turn, goes back through the output transformer and hits your tubes.
- This "flyback voltage" happens all the time in any amp, but they're built to handle it when your speaker impedance is correct. When the speaker has a higher impedance than the amp output, these voltage spikes can become much larger and damage the amp.
- Keep in mind that all of these problems are reduced as you lower the volume. For bedroom playing, you could probably get away with a larger mismatch and not do any serious damage. I can't guarantee that, though.
