Unwanted preamp switching noise often arises from (1) a power switch that causes a voltage spike when it is flipped and (2) uneven discharging of capacitors. A high quality preamp such as the ART could easily avoid these issues by buffering the power switch (or using a well-designed one that doesn't cause a voltage spike) and by using better capacitors or designing circuitry to allow them to discharge without affecting the audio circuits. Obviously, lesser products require proper sequencing to avoid unwanted noise.
