power cords with active shielding

Jim, speaker cables are antennas for the electrical noise since amplifier has low output impedance only for low frequency signals.  Induced noise enters amps input thru the feedback loop.  

As for the ferrite - on one hand you need highest permeability, but on the other high frequency response.  Self Resonant Frequency (SRF) specification might be useful since above this frequency impedance of the choke for common mode signals will drop like a rock.  Read this:
https://www.coilws.com/index.php?main_page=page&id=128

I've been once in the company that had so much of the noise pollution (multiple high frequency seem welders) that nothing worked - not even telephones.  Technicians there told me that common rules, like grounding shield on one end only don't even work. At the end they had to put every power cable in conduit.  We had to put our equipment, we sold them, in the metal NEMA box with metal mesh screen on the front door glass.

I did read everything but unfortunately cannot think of anything else, especially since Al and Ralph already commented.  It could be the power cable, speaker cables or both.  I listen to recorded noise and watched youtube video showing sound of different type of interference.  Judging by >1s gap between occurrences I would suspect that it might be intelligent power meter.  They repeat readings from each other for each household or apartment on "Mesh Network" at 900MHz few times a day.
Where is it located?  It could be also that it picks up all the sources - cell tower, trains, power meter etc.   What about getting metal conduit in Home Depot (it is cheap) and cut it to the size of power cable and speaker cables.  After running cables thru it ground them well.  Look for alternative grounds like faucet or radiator.  It is only to check what it is.  If noise goes away then we can start removing conduits one by one.  If it doesn't then either filtering from the Furman is too weak or amp itself is picking up noise and needs better grounding.  Most of shields are not perfect Faraday cage and have to be grounded in order to work.  For instance to make high frequency current flow on the outside because of skin effect (cable shield or metal box) it needs place to flow to - a ground.  Amplifier box should be grounded with separate wire directly to good ground.  I would experiment with different grounds.  If faucet pipes work, than running long ground wire is better than suffering from audible noise.  Perhaps speaker box also needs shielding? (Shadorne suggested it).  If it is difficult to do/check replace it with very small speaker (even plain individual) and shield it.