The easy answer is the more current required, the lower the gauge wire is needed. It is that simple. With that being said, it does not hurt you to use a low gauge power cord on your preamp. IMHO, it will be difficult for you to tell the difference sonically between properly sized power cords as the power supply inside of each of your components has more more influence over the sound of your component than a power cord. This is especially true if your device has a switching power supply where the power is regererated at a higher frequency.
pre vs amp power cable question
would a linestage or phono preamp ordinarily "require" a smaller gauge power cord than an amplifier, where the current draw is potentially much higher?
for that matter, since an 18 gauge cord will pass more than the rated current draw of most amps (c. 2.5 amps? - please feel free to correct whatever misconception is implicit in this) - does an amp "need" a "bigger" cord than, say, a 14 gauge cord, which would seem to be sufficient overkill? Or is it more about unexplainable filtration effect or something else? Or size doesn't matter, all you can do is randomly experiment?
not that performance differences between power cords compared to the Romex in the wall makes much sense, but still...
for that matter, since an 18 gauge cord will pass more than the rated current draw of most amps (c. 2.5 amps? - please feel free to correct whatever misconception is implicit in this) - does an amp "need" a "bigger" cord than, say, a 14 gauge cord, which would seem to be sufficient overkill? Or is it more about unexplainable filtration effect or something else? Or size doesn't matter, all you can do is randomly experiment?
not that performance differences between power cords compared to the Romex in the wall makes much sense, but still...