loud speaker impedance

A loudspeakers impedance is everything but consistent across its frequency range. That is why when you look at a loudspeaker's specifications it will say something like "NOMINAL" impedance 8 ohms. Nominal means, somewhere around, mean, about, average, could be, that's what we want and wish for you to believe, it is thought to be or just about anything. In fact a loudspeaker impedance may have very different impedance values at various frequencies. It means 8 ohms ain't. You should check out the impedance versus frequency response of the loudspeakers you are using to be sure.