If you look at a power amplifier power supply design, they don't use regulator therefore it is much more susceptible to voltage change and variation. Most amps (if not all) only use only a rectifier stage and use output capacitor to try to keep the voltage as constant as possible. Because of this power cords are very important to a power amp.
On the other hand most pre-amp (if not all) use regular (with feedback and all that), so it is not as susceptible to voltage variation so therefore not as sensitive to power cord.
On the other hand most pre-amp (if not all) use regular (with feedback and all that), so it is not as susceptible to voltage variation so therefore not as sensitive to power cord.