Less volume with a more powerful amp

Amp gain (sensitivity), not output power, determines the necessary level setting.

I do not think there is a single answer. Since it is interdependent with other design parameters, it is not an independent variable.

Obviously, sensitivity needs to fall within a useful range for commercial success but some designers have specific targets and others do not.

The significance of the gain and of the SNR is dependent on what you feed it. I do not know what you mean by "the threshold of signal loss."

In theory but, in practice, it would depend on the output capabilities of the preamp.