Game Changing Tweak

Bemused, I think that you will find that it's a matter of degree. The equations for filters are well known, see for example a Radio Amateur's Handbook from the 50's. You will note that EVERY LRC filter filters EVERY frequency - the question is, how much.

Sure you can use a single phase motor to drive a three-phase motor backwards to get three phase, making filtering easier - but you choose your poison. I chose brute force LRC with single phase. YMMV

There is no "bad" anything.
In fact, 

The ADD-Powr idea is not about filtering noise.
It adds signal, and does not filter  nor address noise.
Do you remember the term signal-to-noise ratio, or S/N?
If you increase S what happens to the ratio?
My math says the ratio gets larger.

Go get yourself a dB sound level meter.
Play pink noise or white noise into your system.
Send it as mono or stereo feed.
Take said meter locate and position and point it at:
1 - 2 feet from each speaker
Repeat from 5 feet.
Repeat from the sweet spot between speakers 
Record the measurements.

Plug in an ElectraClear or any ADD-Powr product, and repeat the above.
End of story.

Bemused, have you done this experiment? What was the result?

Geez, where were you guys on the thread about improvements from AC power cords?

Current harmonics occur on power lines due to non-linear loads, of which there are many.  Do they affect the sound coming out of your system?  I read through ADD-Powr's website. They claim,
"A complex wave "disturbance" rides  along the AC line. Since it is a low frequency signal, it is not filtered away by power supplies, such as a/v component power supplies.  Instead, it passes through the filter stages and becomes part of the DC reference supply voltage."

What low frequency signal are they talking about?  They don't say. This claim seems bogus to me, if for no other reason than they don't publish any actual data.  I couldn't find any product specs.  And, if it were a real thing, it would be easy to demonstrate.  Specifically:

1.  Show me a frequency graph of incoming A/C power (both the 60Hz  and whatever harmonics you want to include) and the corresponding frequency graph of amplifier output.  

2. Now introduce the "device" and show me the same graphs.  This kind of before and after will show both the frequency domain and the scale of the impact of the high pass filter.

Nada.  Nothing.  A picture says a thousand words, but only if there's something to show.

One other thing ADD-Powr says in describing their technology:  
"

• They generate and propagate low frequency signal information to affect harmonic change to audio signals.  

• They super saturate the AC line voltage with harmonics to affect a 1 to 5% increase of the voltage peaks delivered to the power supplies of system components."

Why exactly would you want to increase voltage peaks?  Stable voltage is extremely desirable, peaky voltage not so.  The vast majority of PQ problems are voltage related, either a sag or a surge, and those can mess mightily with electronics.  Also, how does super saturation with harmonics increase voltage?  Harmonics are undesirable.  Now, if you are doing PF correction, that will slightly lower voltage by lowering current flow, but they also specifically say, "

     ADD-Powr products are

    Electronic harmonic resonators.

    They are not AC power line conditioners".

Honestly, the more I read this on this site the more it screams Snake Oil.