OT... Source impedance?

[J M Fahey]

Using opamp techniques, you can mimic much of the external effects of tube power amps. It probably takes an infinite number of refinements to get them all. 8-)

You are right on both counts.
Yet I think that you do not need to mimic *everything* , but with a reasonable approximation the sound can be *quite* accurate and musical.
I always joke that Mr Pritchard (my respects to him) , one of these days, is going to patent how to emulate a glass envelope, a spirally twisted filament and the smell of a hot bakelite socket.