Hi Ghosthouse,
A theoretically ideal voltage source has an output impedance of zero, and a theoretically ideal current source as an output impedance that is infinite.
Neither exists in practice, of course. However most (but not all) solid state amps have an effective output impedance that is low enough in relation to speaker impedance to enable them to be considered as voltage sources for practical purposes. Provided, of course, that they are operated within the limits of their maximum voltage, current, power, and thermal capabilities.
On the other hand, tube amps have output impedances that are usually significant in relation to speaker impedances, and also tend to differ widely among different designs. Consequently tube amps fall at various points on a continuum between ideal voltage source and ideal current source.
An ideal voltage source will maintain an output voltage that is constant as a function of load impedance (again, as long as it is operated within its maximum voltage, current, power, and thermal capabilities). Per Ohm’s Law, if the load impedance is purely resistive the current that is supplied will equal that output voltage divided by the load resistance. It gets more complicated when the load has a significant inductive or capacitive component.
An ideal current source will maintain an output current that is constant as a function of load impedance, and per Ohm’s Law the voltage it will supply into a purely resistive load will equal that current multiplied by the resistance, as long as it is operated within its maximum capabilities. And again, it gets more complicated when the load has a significant inductive or capacitive component.
Best regards,
-- Al