Usually, the audio signal is represented as a change in voltage at the output terminals of the power amplifier. In theory, that voltage is identical at the input terminals of the loudspeaker.
The instantaneous current through the system is determined by the relationship between the instantaneous voltage and the impedance of the loudspeaker.
The problem is that this model is too simple. The power amplifier has an output impedance (which is typically independent of frequency, more or less…). The speaker wire has a measurable impedance (resistance and capacitance – therefore it’s frequency-dependent) that is a result of the wire’s construction and its length.
If the output impedance of the power amplifier is close to (or worse: larger than) the combined impedance of the speaker wire and the speaker (which is highly dependent on frequency), then the voltage at its output terminals is different from whatever it should be. This is because the amplifier’s output impedance is in series with the wire and the loudspeaker, and therefore the system acts like a voltage divider (measuring at the amplifier output terminals).
This means that the voltage at the power amplifier’s output will not be correct – but since the load impedance (the wire+loudspeaker) is frequency-dependent, then the error will be signal-dependent (since that’s what determines the frequency).
Therefore, if you have a loudspeaker with a low impedance at some frequency, then you need to use a power amplifier with a MUCH lower output impedance at that frequency in order for it to behave.
So, it’s not really the case that the loudspeaker is “difficult to drive” – it’s that the loudspeaker/wire/amplifer interaction makes the amplifer misbehave.
The simplest way to avoid this problem is to buy a power amplifier with an output impedance as close to 0 Ω as possible. This is a good solution if you have an amp in a rack, and you have no idea what will be connected to it tomorrow. However, if you are buying a “closed” system (an amp, some wire, and a loudspeaker) that won’t change, then you don’t need to spend quite so much money, since you can chose the amplifier helped by the knowledge of the wire+loudspeaker’s impedance.
Cheers
-g