Typically higher than 20 kHz.
The problem with a textbook linear power supply is that it is only trying to filter out the 60 Hz line frequency.
Unfortunately the diodes used in the rectifier have to turn on and off to convert AC into DC.
When this happens they ring a bit at a high resonant frequnency.
Guess where this switching noise goes?
In two directions, conducted out of your power amp (or pre-amp or whatever) down the power cord and into your other equipment AND into your power amp or pre-amp or whatever.
The filter capacitors in the power supply may not be able to filter out this very high frequency switching noise (often the power supply capacitors have too much inductance at high frequencies). Power supply capacitors are a subject onto themselves. Some are better at rejecting power suuply noise than others.
The switching noise is also radiated (i.e. broadcast) into nearby equipment.
Measuring this stuff is an art unto itself.
Not every audio company has the budget or equipment or expertise to do this.
The problem with this high frequency noise is that it may be demodulated down into the audio bandwidth.
Audiophile power cords may be designed to reduce all this noise from being absorbed and conducted.
The problem with the audiophile power cord companies is that they are not usually too forthcoming with design and measurement data.
One company that does advertise some measurement of noise rejection is Russ Andrews.
Cardas alludes to this when they mention that their shielded power cords may also include some extra capacitors.
You'll notice that a lot of these audiophile power cords are shielded.
I used to work with some SwitchMode Power Supply designers. The boss "volunteered" me to help them get some EMI/RFI rejection/suppression certification for the North American and European markets. I learned enough to know how little I actually knew about the whole subject. Which is way more than Joe Average audiophile. But it was interesting to see how much crap noise the power supplies themselves generated.
I guess Odyssey doesn't trust some of these power cord and power bar vendors. I would also wager a guess that Odyssey does use some noise suppression techniques in their amps and may have some unfortunate experiences where a fancy audiophile power cord or power bar reacted badly with their amp and increased this noise.
Question for you:
do you notice that some amps have less fog, haze, noise, grain under or inside the music than other amps?
I also recall one reviewer referring to this as "spitch".