Expert Pro sound quality

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Timbre can be (actually is) added/changed by an amplifier having a sufficient level of euphonic distortion
A 3D effect can be added by increasing the background noise (at levels not directly audible) particularly if the noise is signal correlated.

I know this from experiments carried out a long time ago to research why LPs sounded so nice despite the technical weaknesses of the mastering, manufacturing and replay processes.

Many amplifiers, particularly those using transformers and/or low feedback add audible amounts of extra overtones which may make an instrument sound more pleasant to some even if not true to the microphone pickup.

Or one makes the electronics better to lower distortion and noise, in effect to let it get out of the way more to let more of the original source to come through.

Indeed, but a lot of enthusiasts choose to do the opposite, ie choose euphonic distortion, valve amplifiers, particularly SETs, and low feedback being popular examples. Some of the "noise reduction" strategies discussed on forums have no mechanism by which they actually reduce noise, Entek for example, but plenty of people prefer the sound they add.