Brian, there are several different things that PLC's are "supposed" to do. The obvious task is that they should filter out electrical noise. Some of the common sources of noise are listed below. Keep in mind that the noise may not even be generated in your house, it could be coming from somewhere nearby that shares the same pole transformer.
Pops and ticks from light switches, timers, thermostatically controlled devices, etc...
Hash from electric motors found in your fridge, fans, power tools, etc...
RF / digital noise from CB's / Ham radio's, clocks, microwave ovens, computers, etc...
Noise can even be generated by components WITHIN your audio system. I just talked to someone the other day that told me that their tuner / cd system interfered with each other. If they wanted to use one, they had to have the other turned off. I also know that many digital based products, especially amps that use switching type power supplies and cd players in specific, can pump a LOT of junk back INTO the AC line. Anything plugged into that line then gets to share the noise.
By using a "decent" Power Line Conditioner ( PLC ), parallel line conditioner ( RGPC, Audioprism Quiet Lines, etc... ), Isolation or balanced power transformers, true power regenerators ( PS Power Plant ), etc..., you can minimize the above problems and even isolate some of the components from each other in terms of AC crosstalk. Many different designs have several different filters within one chassis. This allows each component to have filtered power coming into it. It also filters what can go back INTO the line, which means twice the the filtering action and reduced interference / component crosstalk within the system itself.
What does all of this "hoopla" buy you ? If properly done, you can gain a much quieter / blacker background due to a lower noise floor, increased dynamic range, increased system resolution, more "liquid" presentation, increased separation of notes / instruments, less grain and glare, greater bass impact and definition, etc...
On the other hand, using low quality PLC's or using a PLC that is not appropriate for that specific application can result in squashed dynamics, strange soundstage effects, etc... Most of this would be due to current limitations ( long term or momentary ) within the filter itself. That is why many people tend to filter the line level equipment but plug their amps directly into the wall. This gives them quite a bit of "filtered protection" and avoids most of the problems associated with current limations.
Keep in mind that everything surrounding power line filtering is a band aid to a much bigger problem. You have to remember that all we are doing is "shaping" the AC waveform coming out of the wall and converting it into a musical signal via our audio equipment. Since 5% - 10% distortion would be considered "insane" in a "high fidelity component" ( except for speakers ), take into consideration that this is the type of signal that we typically feed each and every component in our system. Those are the levels of AC distortion that one can normally find coming into your dwelling if you live in a populated area.
As such, nothing short of TRUE power regeneration ( PS Power Plants* ) will give you "optimum" results. Those are the only devices that i know of that actually produce a pure and clean signal to work with. Others might make claims of "regeneration", but i've yet to see one that did. Unfortunately, "optimum" is typically equated with "state of the art" and anything that is "state of the art" typically costs a good chunk of change. A suitable Power Plant for a good sized system is no exception to that rule.
While a filter can remove most of the additional "junk" that is added to the AC supplied by your local power company along the way, it can't correct the deficiencies that it starts off with coming from the source. As such, one can see where removing 5% - 10% distortion right off the top might make a bigger improvement than trying to remove a small percentage of that through cumulative efforts ( various power line conditioners, power cords, parallel filters, etc... ).
Obviously, the first thing to do before shelling out tall cash would be to check into the power supplied to your components and see just how "dirty" it really is. This can be done by using a very simple yet effective device made by Audioprism called the "Noise Sniffer". You can literally listen and hear all of the junk coming into the AC system. You can do this outlet by outlet, circuit by circuit and hear the differences. You can even track down some of the "offending" devices and repair / replace / modify them as necessary. If you didn't have a bunch of noise, you could probably get away with a small amount of filtering. As noise levels escalate from there, so do the costs and complexity of the filters necessary to get the job done.
Keep in mind that many of these changes are cumulative and very subtle in nature. One can make a bunch of small changes that would go un-noticed by themselves yet might add up to something substantial when combined. That is the approach that i've been taking since i haven't had the money to dump into a more thorough "all in one" approach at one time. Sean
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*For the record, i have no personal or business affiliation with PS Audio nor do i own a Power Plant. However, i do admit that I would like to purchase one in the very near future. My comments are based on the technical aspects of how various designs work. Obviously, results may vary due to a multitude of potential combinations and variables. Always "try before you buy" within the confines of your system whenever possible. Like i said, there is NO sense in buying something that you don't need.
