That should probably say "low impedance". What happens on some speakers is that the impedance the amp sees falls way off when trying to reproduce LF signals. So the amp is forced to try and keep up by supplying more current. If it goes too far the amp clips and that's not good.
My Aerial 10t's are a great example of this. I believe these can fall to 2 ohms or lower and will play down below 20 Hz. You can drive them with 50 watts, but don't turn the volume up much at all. Now, hook up 200-300 watts or more from a high-current delivering amp and they sing beautifully.
It's not easy to tell from efficiency numbers, either. I'm not sure what the numbers are on Doug's B&Ws but I'm guessing they don't drop impedance too much even though they are relatively inefficient.
My Aerial 10t's are a great example of this. I believe these can fall to 2 ohms or lower and will play down below 20 Hz. You can drive them with 50 watts, but don't turn the volume up much at all. Now, hook up 200-300 watts or more from a high-current delivering amp and they sing beautifully.
It's not easy to tell from efficiency numbers, either. I'm not sure what the numbers are on Doug's B&Ws but I'm guessing they don't drop impedance too much even though they are relatively inefficient.