This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
Each home and each outlet has slightly different power line
impedance and power line noise. Each amplifier is affected by
power line impedance and power line noise differently. Power
line conditioners try to reduce this line noise. Some also
change the power line impedance in a way which is supposed to be
better. We will leave it to your ears to decide if these
devices help the sound of your system enough to justify their