Should the record producer be allowed to adjust the electronic component values (between safer range of values) of a simple say front end mic amplifier etc circuit?

At your risk. For example adjusting just Vb on an open control element amplifier. For example an open grid electron tube front end microphone preamplifier. Or even adjust cathode temperature by adjusting heater voltage. Other example, adjusting Rl, Rk, Vb in order to suit her, his recording environment and the sound in their mind. In pure low component count simple signal path circuits it should be possible using variable resistors, capacitors, inductors etc. Would this be a better alternative to the fixed components world set by the electronic engineer who then has to add for the producer, lesser perhaps sounding EQs, adjust threshold in a limiter, overload protection pad etc. When for example Rl is adjusted the centre quiesecent point can be shifted for production of less or more second harmonic distortion and less or more 3rd etc. Vb can dictate headroom which may be intentionally set to low for more processing of peaks. Anode current, usually in microamps region, cathode temperature can decrese gain but improve signal to noise ratio or even increase gain. Interaction of change of gm (transconductance or anode resistance) can change the frequency response by interaction with the output transformer inductance in order to create a flat frequency response from producer's mind to listener's mind. Should the circle between record producer and electronic engineer complete in phase or out of phase;) See also: Adjusting cathode temperature of electron tubes, for example Georg Neumann may have deliberately underheated the VF14 electron tube of the U47 microphone from %0mA at 60V to 40mA at 35V.