True, but as long as the load impedance is within the amps ability to drive it the amp doesn't care. Differing impedances would bother a constant current amp, so amps are constant voltage devices for just that reason. Besides, impedance is always a nominal figure. The typical in-use impedance of an 8 ohm woofer actually runs from a low of perhaps 5 ohms to a high of 50 ohms or more, depending on the frequency at which you measure.