Should a linear amplifier be considered as a current source or a voltage source?
From the Thevenin and Norton theorems we know that any source (aside from an ideal voltage or current source) can be considered as either a voltage source (Thevenin equivalent) or current source (Norton equivalent).
Practically, if the source has a low output impedance, it will behave more like an ideal voltage source (its output voltage will be insensitive to the load impedance) and it's usually more useful to describe it as a voltage source. And if it has a high output impedance, it will behave more like an ideal current source (its output current will be insensitive to the load impedance) and it's usually more useful to describe it as a current source.
A class B amplifier is designed to have a low output impedance, therefore we will usually describe its output as a voltage source.
Even a class A amplifier, which is a common emitter or common source stage, and has a higher output impedance than a class B amplifier, usually has a lower output impedance than the load it's designed to drive, so it will usually be described as a voltage source.
If you want an example of a current source, look for things like current mirror circuits, or op-amp circuits designed to deliver (near) constant current to their loads.
All of these are linear circuits. But that is a very wide category --- basically meaning the output is proportional to the input --- and isn't restricted to only voltage sources or only current sources.