First bear in mind that the output power specs on modern amplifiers are what they can sustain short-term, typically a few tens or hundreds of ms. This is fine for most music which has a high crest factor. If you're running EDM subs, a shaker table, or using the amp as an AC power supply, you need to de-rate it considerably. In a "serious" application you likely should talk to the manufacturer's engineers for guidance. The other simplification is that pretty much all specs are given for a resistive load. Loudspeakers are reactive and different amplifiers may behave slightly differently even though they have the same spec. (Back in the day some amplifiers would even become unstable with too large a capacitive load.)
Audio amplifiers are indeed voltage amplifiers and have an essentially zero output impedance in the small-signal or linear sense. (The exception is certain amps that allow for a negative output impedance to compensate for wire resistance on subs.) The available peak voltage (large-signal), however, tends to sag a little with increased current. This is because there are resistive losses inside the amplifier that cannot be compensated for by the negative feedback that is used to fix the small-signal output impedance. You can see this on the power vs load-impedance specs that are not exactly consistent with constant voltage even when the amplifier is not current limited. This small sag can be ignored if all you're trying to do is get an idea of the optimum load impedance, which will occur when the voltage and current limit simultaneously.
Zopt = Vmax / Imax
then
P = Vmax * Imax
--Frank
