This is very interesting question. It seems simple to answer, on the surface, but when you get into it, not so simple. In fact it can get quite complex. I will throw out some tidbits and let you guys run with it.
1. Think of the amplifier as the utility company and the speaker as the only load they have.
2. The speaker is an electromechanical device, a transducer. It converts electrical power (or energy if integrated over time) into mechanical motion which results in a pressure wave, which we perceive as sound.
3. The load is an AC Load, so it has AC Resistance (will hit on this later), Inductance and Capacitance. because of its reactive components, it has the ability to STORE energy and RELEASE (back EMF of the speaker) energy.
4. DC Resistance implicitly indicates 0Hz operation!
5. When you talk about any AC Load you MUST specify the frequency. Once you know the frequency the reactive terms have meaning. You can calculate the magnitude of each one and then vector sum them together to get the net reactive impedance, either inductive or capacitive. You then vector sum the AC Resistance with the resulting reactive component to realize the impedance vector Z at the frequency of interest. You then can compute the Phase angle and determine the true power and apparent power.
6. AC Resistance takes into account the Skin Effect present in all non-zero frequency wire situations. It is a calculation that is always done by power transmission engineers AND especially RF engineers and/or anyone who is subject to its effects. The best analogy I can come up with to describe this Skin Effect is the water pipe analogy. At DC the full cross sectional area is usable. As the frequency goes from zero to non-zero another solid pipe, small in diameter at first, is inserted into the larger hollow pipe diameter. This forces the flow to the remaining open area, thus causing an increase in resistance. As the frequency increases, the solid pipe get larger and larger and forces the flow closer to the surface of the wire. The cross sectional area gets smaller and smaller until very little of the original cross sectional area is available. As an example at 50Hz 100 ft of 16 gauge wire has .39085 Ohms of AC Resistance. At 60 Hz its .39086 Ohms, at 600Hz its .39108, at 6000Hz its .39328, at 20kHz its .42022 Ohms. Anyway you get the picture. The inductance, the number of wires and their spacing will also come into play. The formula can get quite complex. Knock your socks of if you want to delve into this area.
