Pepco is like a lot of other utilities. Having loads on the grid that shift the power factor away from unity causes them a lot of problems so they install banks of switched capacitors to adjust the power factor. I don't know all the details but if you look at the poles in your area you might find a box connected to the high voltage lines. They must have some circuit to detect power factor and add or remove capacitors. These were probably used to balance inductive loads like motors and transformers that have leakage inductance but I wonder how the plethora of switch mode supplies affects their assumptions.

It is most likely distortion rather than dc on the line. There could be dc current on the line if you have some half-wave rectifier-powered stuff, but it should be low power. The pole transformer would largely remove any dc voltage appearing on the line.

Power-factor correction is basically of two forms and addresses two different issues. Traditionally, power-factor correction involves placing additional reactive components on the power-distribution grid to alter phase to compensate for (generally) inductive loads such as motors. They usually use switched capacitors for this. This is less of a distortion issue than a phase issue.

The second issue for power factor is electronic loads. Diode rectification into capacitors causes current to be drawn in pulses, and this puts a lot of harmonic distortion on the line. This causes a lot of additional power loss, even to the point of severely reducing the amount of power that can be drawn from a (line) circuit. Line harmonics are of such a concern to the power company that end-use power-factor correction is now required in electronic equipment that draws above a certain threshold. This was 75 watts in the US some time ago, I don't know if that number has been made even more stringent recently. Most if not all of the new LED lamps include PFC even though they use much less than 75 watts. If you have a bunch of them, this is a big deal. I have been including it as standard in my amplifiers for about 10 years now. This type of PFC uses electronic means to reduce the distortion by causing the current to be drawn in phase with the voltage and to cause the current waveform to be reasonably sinusoidal rather than pulsating.

I put a significant effort to figure out how to do PFC, even though as a small manufacturer I don't think anybody would really challenge me for not having it. To me it makes sense not to put this distortion in the line where your audio system is because it is likely to affect other equipment in your system. At the very least you could expect buzzing transformers as noted above.

David

Glad the SMPS are working. Those amps are beasts.

The common thinking on line is you have a DC component on the line. .

At line frequencies or other low frequencies the best shielding is distance. That is why I try to put transformers in a separate housing away from the audio circuits. In with the transformer is the rectification and one stage of filtering so I am sending DC over the interconnect which will reduce radiated energy. If you can't do that then placement and orientation of the transformer can help. If none of those work, high permeability materials such as mu metal can be used. Here is a useful link:

https://en.wikipedia.org/wiki/Electromagnetic_shielding

Noise of any sort in a system is one of the biggest PITA. As Walt says you just have to gut it out. Last weekend I had a friend put a new piece of equipment into his system and he had a noise problem that was not there when he had it on the test bench. Imagine that! Something worked differently when introduced into the system. I told him that he would just have to look at all the possible points that noise could enter but there was no way I could diagnose it remotely.