The electronics should be run at the highest possible gain, at a given supply voltage, where feasible.
I don't understand. How would you lower the gain of a preamp without major reconstruction? THD comes from operating an active device in a non-linear manner. The operating point (bias) of the active device determines where you are on the curve and this isn't really related to gain so THD really isn't related to gain except in the sense that it will increase if you allow the signal to become so large it swings into the non-linear portions of the device's curves. It's a bit more complex than that since bias and operating points can affect gain but that's the basics. Maybe I'm missing something here.