Few very different concepts being discussed, here is what i have found as it may help.
Passive filtering: Really intended for EMI supression are those IEC filter box's as shown in your last link, found inside just about every computer power supply to prevent noise from back polluting the mains supply. Really intended to filter out high frequencys caused by switching power supplies but can also be useful to filter out differential and common mode noise, i.e. if you hear clicks and pops as electrical devices around the house are switched on/off, these can remedy that issue. Inside they are basically a CLC filter with a common mode choke.
I have built a few DIY versions, and recently bought a Belden Pure AV box which has a bunch of those filters side by side each feeding a bank of power outlets. Theory being each outlet is isolated from each other, so you put noisy devices like TV's and set top box's on one channel, and analog audio sources on another etc.
Many people claim these filters are a bad thing for high current applications, i.e. big amplifiers that consume a few hundred watts. Belden even have a "high current" channel on the far right with no chokes for that purpose.
Overall, none of the above will improve the sound of your system in any way unless you have a problem to start with, if you have nice clean mains, well thats as good as it can get.
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fcgim.audiogon.com%2Fi%2Frv%2Fs%2Ff%2F1164293595.jpg&hash=556d696136917579f05816c381f2a6f372278cab)
Active Filtering: Or intelligent filtering like the PS Audio Power Plants Grainger mentioned are basically big amplifiers that recreate a nice symmetrical AC wave form. For example my signature pic is actually a capture of mains power during the day, note the flat top and bottom of the AC waveform, a Power Plant in theory will correct that and recreate a nice sinusoidal wave form. Why that matters i am not entirely sure.
Balanced Power: I built a balanced AC supply a month or so back and i'm still testing/experimenting with it. The unit i built has a a big 1kva toroid that takes 0-240v on the primary, and outputs 120v-0-120v on the secondary. In theory it also has some filtering benefits as not all noise frequencies will pass through the transformer. Main difference i have noticed are quieter running transformers down stream from it. For example my F4 amp has a big 500va toroid which doesn't buzz half as loud running off the balanced power source, and the same it also true for the transformers in my valve amps. Cant say for sure if the valve amps sound any better as i haven't done any measurements.
DC Filtering: Another topic that will arise searching for mains filtering, and something i had to implement into my balanced power supply. If you have any DC offset in your mains it can cause transformers (specifically torioids) to saturate and hum. In my case the F4 and balanced supply toroids used to growl angrily during the weekends. I added a DC filter circuit via a big switch and sure enough it cures the noise when the need arises.
My current setup is the Pure AV box feeding the DAC's, computers, TV, set top box all on different channels. High current channel then feeds into the balanced power supply box, which in turn feeds my F4 amp, S.E.X. amp, Preamp, and a couple of other valve amps.
