This is all the result of trying to explain thing using language that is not suited to what you are trying to explain (as I tried to explain elsewhere).
I suppose the difference between regulated and none regulated supply is the operation point, which is held constant with regulation
Not really, remember that above a certain frequency the cap at the end of the power supply is working to decouple the supply and will function to (within limits) fix the voltage at that point. The fact that you can measure with a meter that the addition of the load changes the voltage from the power supply is just because you are measuring the DC voltage at that point, and there is no cap big enough to sustain the voltage at DC. All the regulator is doing (if it helps you to picture it) is behaving like a infinitely large decoupling cap, and no one has considered getting worried about that cap altering the operating point because the voltage across it does or doesn't change.
modulated by the signal 'on top ' of the operation voltage.
I think its this "on top" thing that is the cause of your confusion there is just the way it is now at this instant, and then there is the way it is at the next now an instant later, and so on, and also as I said at Owston, you cant just think in terms of voltage you also have to think in terms of current AT THE SAME TIME. I think its the "same time" part that trying to think about it with words where the problem comes as the words allows an abstraction from reality that means that the link between the description and the reality is meaningless.
its just in my mind a totally fixed operating point that includes the signal, is not how a valve was designed to operate... or is it
A valve was not designed to operate, a valve was invented and then we found how it operated, you have causality the wrong way there. Remember the most important thing
A valve is only aware of the voltages and currents on its pins it knows of nothing else. This is also true of every other electronic component. If you understand that
and what it means it will all make sense. If you don't it never will.
ps. there's quite a lot of difference between series-reg and shunt-reg in terms of what happens in the PS, to achieve that aim.
Though in the context of this attempt at an explanation that difference is of zero importance, the power supply can be considered as a perfect voltage source in series with a resistor and a cap after that resistor. All that varies (that matters to this discussion) if you use or don't use a regulator is the size of the resistor.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.