Right. The problem you have with AC power is the 5th harmonic, so for 60Hz in the US we are talking about 300Hz.
This is caused by distortion of the line, usually by a power distribution transformer somewhere upstream of the audio system, often hanging on a power pole. The 5th harmonic is far more important than spikes, high frequency hash and the like. It is responsible for power transformers becoming suddenly or intermittently mechanically noisy, rectifiers for radiating diode noise and for contrary forces developing in AC synchronous motors.
The 5th harmonic is hard to filter out. It requires large chokes in the line conditioner or perhaps a servo means around an isolation transformer (the Elgar some of you may have seen me mention is an example of a conditioner that uses the latter principle).
As is apparent, once you understand this, many AC conditioners do not touch the 5th harmonic. Now this is not the same as not doing something audible, and in some cases there may be no 5th harmonic present and a conditioner that is filtering only higher frequencies might still have an audible improvement to offer.
A secondary concern is loss of line voltage through the conditioner with its attendant power cords and AC connections. With particular reference to the MA-1 at the source of this thread, line voltage drops of only 3 volts at the IEC input of the amp can result with a loss of as much as 40% of full power. If you have ever wondered why an AC power cord can make an audible difference, you don't need to look much further than this simple fact (although there are other factors too, but that is for a different thread
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Now imagine a system with a power conditioner that is otherwise found to work quite well. Every component in this system is plugged into that one conditioner. In the system I have in mind, mentioned elsewhere on this website, we had a set of MA-1 monoblocks, a pair of high power solid state amps, an active crossover, preamp, CD player and turntable. The total power in this system at idle is over 10 amps, and all of that is coming through one power cord that is used by the power conditioner.
Given the facts that I have presented here, what is going to be the overall effect? At the very least, a loss of line voltage. This is unavoidable- the power cord itself may not see much voltage drop on its cable, but there are mechanical connections that after a few hours are warm to the touch at either end- the connector in the wall is playing a role as well as the AC connectors elsewhere- all contributing to voltage drop (hence the heat they make).
So the bottom line comes to two issues: how well the conditioner can filter the 5th harmonic (and if no 5th is present, you are lucky) and the voltage drop the the use of such encourages.
For this reason I usually recommend that anyone with a larger amplifier (especially a larger tube amp) plug the amp directly in the wall, or at the very least, audition the difference!
