Except for some speakers like the Magnepan etc., most speakers have a fairly variable impedance with frequency. This information is not given with the user manual or on most review sites. Stereophile is one among the few that do provide this information.
"Typical "" impedance is usually the impedance at a low frequency . 6 ohms would mean that it doesn't go below 6 ohms at 'some' lower frequency !What happens at high frequencies is not mentioned. Sometimes this is of no consequence but at other times it does matter as it does drop very low and stresses the amp.
At low volume this shouldn't matter much but as you increase the volume it can cause different types of problems. Heating up of the amp or blowing fuses or components. Often before all that it will start to sound worse and strained. That's when you have to turn back the volume.
I've attached the impedance plot of three speakers .
Dynaudio Excite X14 rated as 8 ohms in the spec sheet.
The other one is an electrostatic speaker Acoustat Spectra 1100 .
The impedance is rated at 6 ohms nominal ( but they do say 2 ohms minimal !) Not all brands say that.
The Wharfedale Jade speaker is rated at 6 ohms but will be a difficult load for an average amp !Look at the impedance graph. It goes below 4 ohms !
You can check them out yourself at :
Dynaudio Excite X14 loudspeaker Measurements | Stereophile.com
Acoustat Spectra 1100 loudspeaker Measurements | Stereophile.com
Wharfedale Jade 7 loudspeaker Measurements | Stereophile.com
( left side Y axis is impedance in ohms .. The dark solid line).
While many manufacturers don't give the whole picture regarding impedance, they often will say that the speaker needs a powerful amp to drive it properly!