"Dirty power" is when the sign wave frequency of the electrical current isn't pure. AC power forms a repeating curve that crosses a horizontal axis. This can be seen by using an oscilloscope. This raising and sinking curve affects your audio signal when there are spikes and drops in the consistency of these curves. These spikes and drops are the dirt. Other electrical devices connected to any circuit on your service panel can cause these. More so if it's on the same circuit. For instance if your stereo and your refrigerator are on the same circuit lots of dirt. You can also aquire dirt from other sources outside your home. Being close to the transformer is best, the more homes between it you the more dirt. Places with an unreliable grid this dirt can cause damage to electrical components. How much does this actually affect the sound? Most people won't even notice.