Flat mic to flat preamp, flat response, Yes?...

No.


It depends.


One factor is the output impedance variation with respect to frequency.


And the input impedance variation wrt frequency.


Other phenomena too, such as proximity effect, directional characteristics and frequency dependence...


And of course all those psycho acoustic effects that make response non flat from singer;s vocal chords (or even singer's or producer's brain ) to listener's brain.


So the best judge is the listener's mind or brain.


And we aim for flat frequency response from singer's vocal chords to listener's brain.


Intentionally introduced non linearity in impedance can be used for this aim.


For example a Pleiades filter in parallel (shunt) with the microphone's output terminal, (an inductor or coil as used inside an Electro-Voice RE-15, or RCA 77DX,  (at your own risk),


The decreasing impedance of the coil wrt frequency reduces the output of the mic (energy conservation principle), restoring increased bass perception to natural bass to listener;s mind.





Reference:


Microphones - Lou Burroughs (Electro-Voice)


Flat frequency response from singer's or actor's vocal chords to listener's brain, Sound picture recording and reproducing characteristics - Loye, Morgan - Journal of the motion picture sound engineers