Ah the ohm's law gets chucked in.

I agree totally. But ohm's law is really only pertinent for static loads purely resistive in nature (heaters/incandescent lights etc.) Once you start to account for magnetic fields, variable frequency, net reactance, efficiency(power factor), linear torque, counter torque, inrush, etc. you have to start looking at motor/transformer theory.

Stricktly speaking I=E/R is not used outside of AC\DC purely resistive circiut calculations... The "I" in this case is the "in phase" current relating to the resistive components in a circiut only. It is not what the load would draw from the source.

Resistance and impedance are not to be confused. Resistance is fixed and does not vary with input voltage or frequency. Reactance is affected by the frequency of the applied voltage, and the net inductive\capacative reactance component (leading or lagging Ohms.) Impedance is then calculated by adding resistance to reactance using trig. Since speaker drivers are a combination reactive\resistive load they require more apparent power (va) to operate than what is acutally used to drive the load mechanically (true power--W)

Speaker drivers can be seen as a motor that has a constantly changing mechanical load. The inertia created by the driver must be overcome, quickly, by a sudden inrush of current to realign magnetic fields and change the driver's motion. Inrush in a circuit usually results in a brief voltage sag-- especially when a transformer is involved (and exhibits poor voltage regulation.) This effect can be observed in your house as your lights dim slightly for a second when the microwave starts etc. Car audio is a good example of this. Large capacitors are often used to mitigate the issue.

A good amplifier is able to compensate for these demands by employing an ample transformer, with good voltage regulation and secondary current rating. They will then usually pair the transformer with a capacitor bank to help with maintaining output voltage. This all relates back to Lenz, Faraday, Henry and others that followed Ohm.

The difference and what everyone seems to ignore is that the capacity of the transformer to provide the current (VA rating) is of high importance-- and often cost.