Your scenario should separate linear model from a model that includes limiting values of voltage and/or current. The amplifier is more voltage source than a current source within its linear range...
If the amplifier pumps twice the current into a 4 ohm load compared to an 8 ohm load, then it is a voltage source. A high-quality amplifier will have a much lower output resistance, compared to the resistance of the load (speaker).
When you turn up the volume, you run into limits: a voltage limit (mostly set by the DC supply for the power amp) and a current limit. A voltage source has difficulty supplying enough peak current into a small load: when you add speakers in parallel, more current must flow.
An example audio power amplifier:
An amplifier has a damping factor of 180, which basically means that its output resistance is 180 times smaller than its 8-ohm load. Such tiny output resistance is achieved with a great deal of negative feedback in the amplifier. Within the voltage and current limits, the amplifier runs linearly and the model is a simple voltage source, with 44.4 milli-ohms Thevenin output resistance.
But the power supply likely sags when more current flows - DC supplies are often not regulated.
And the high-power output stage is not perfectly efficient when more current flows, so the peak current limit decreases for small load resistors (like 4 ohms or 2 ohms). Feedback cannot correct these voltage and current limits.
When voltage or current limits are reached, distortion increases greatly. So a power amplifier is specified for maximum power at some (small) distortion. The example power amp is tested for maximum power where distortion has increased to 1% distortion (THD):
- 250W... 8 ohm
- 410W... 4 ohm
- 600W... 2 ohm
Note that halving the load from 8 ohms to 4 ohms doesn't double the power from 250W to 500W, and similarly at 2 ohm load, power is less than 1000W. This is a result of limiting of voltage and/or current.
When operated within its linear range, you can assume the simple voltage-source model:
simulate this circuit – Schematic created using CircuitLab