power cords with active shielding

James1969, Yes, twisting signal wires will reduce it - if it is coming as common mode.  In the case of power cord your signal wires are hot and neutral (return).  Pitch of the twist should be many times smaller than wavelength (17" for 700MHz) Twisting them with 1" pitch should be fine but I would twist as tight as I can.  Noise might be getting in as common mode or normal mode.  Transformer should be good defense for common mode as well, but there is capacitive coupling between primary and secondary.  For that transformers have grounded shield (between windings).  You could also attempt to use common mode choke, winding power cable around toroidal ferrite core.  That would create inductance in series for the signals flowing in the same direction in both wires (common mode) but would be zero inductance (cancellation) for normal mode (signals flowing in opposite direction).  Unfortunately it is difficult to do (connectors, thickness of cable, etc.)  When noise already is or converts to normal mode you can't do much except for filtering.  Electrolytic caps inside present high impedance at these frequencies because of their high inductance.  Placing low inductance cap in parallel would help, but might create parallel resonant circuit with said inductance.  External filter should help but it might create big voltage drops since most of the gear takes power in short current spikes of very high amplitude.  That would appear as reduced dynamics of power amp.  I bought Furman Elite 20PFI conditioner that has power factor correction.  It supposed to present resistive load to the mains.  Other than very tight non-sacrifitial over/under voltage protection it has huge inductor and huge capacitor to deliver required current spikes to the load while presenting constant load to the mains.  With my amp (150W) it does not reduce dynamics. 

At 700MHz shield should be a good defense against noise, since anything induced would flow on the outside of the cable - shield, because of the skin effect.  The only problem might be shields inductance.  I suspect that better cables have better shielding in that respect.

@blindjim

I inserted an isolation transformer (Furman P-2400 IT) into my system, so everything plugs into the isolation transformer. This had no impact on the specific noise I am targeting, but it did have a big positive affect in sound quality - removed any noise I was hearing in the music, and effectively lowering the noise floor quite a bit. But it had no affect on my intermittent noise. I called Furman directly and spoke to one of their reps and he was shocked to hear the Furman had no affect on the noise problem I am experiencing, he then said it was air born, not in the power line. Which I agree with him.

So I powered down my entire system (and apartment) for that matter and left the dedicated circuit on for my system. I put an RCA shorting plug on 1 amplifier’s input, I put a basic power cord on the amp and turned the amp on. I was able to affect the noise when I lifted the power cord and moved it around and it behaved like an antenna. So I am thinking of trying a new power cord to see if that will improve/change the situation.

I was able to identify the 700-900MHz frequency range by using this device:
https://www.amazon.com/gp/product/B007L09DLY/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
It will play the frequencies it’s detecting through the speaker, and I was able to correlate the intermittent noise to the 700-900MHz frequency range which is a dedicated cellular band for emergency first responders. So that made more sense, because when I hear sirens or the subway below, I usually hear the noise.

What are your thoughts?

@kijanki 

Thank you for that clarification.  I too got a Furman device, I got the isolation transformer, the P-2400 IT.  It basically removed the noise floor from the music in my system.  I am still getting the intermittent GSM signal in my system, that is why I am exploring power cables with better shielding.

James1969,  It is possible.  I had for few days CB like transmission loud voice coming thru pretty much everything at home - TV (turned off in stanby), speakerphone, radio.  I noticed every time the same unmarked car with many antennas passing by.  I suspect that his transmitter was out of whack.  FCC intervenes only when somebody complains.  Intensity of electromagnetic field drops in linear fashion with distance. When you are close it can be pretty strong.

@kijanki 

I have a police station down the street, and a fire station down one block from me.  So the public safety GSM band would probably be in high usage during the daytime (like what I experience now).  I'm up at early hours to enjoy my system (when the city is asleep) and there are no sirens to be heard.  The subway still comes and sometimes I hear the noise, we have cell phone service now down in the subway system.

I wonder if the FCC would listen to my case?