Speaker cables - cables make a difference

I usually completely eliminate the noise floor by turning off my amp. 

Good one -- and now I can stop leaving the house, which was my method and very inconvenient, I might add.

I have a set of Harmonic Technology Magic speaker cables that were in storage since I bought my Vandersteen 5A speakers nearly 20 years ago. They sounded great in my previous system, but the spades were too wide to fit inside the barrier strips on the 5A's so I replaced them. About 6 weeks ago I had the spades on the speaker ends ground down by Audio Connexion in Burlington, Ontario (no affiliation) slightly so they would fit inside those barrier strips. Audio Connexion did an excellent job. If I didn't know they were trimmed a bit I'd never know it was done. Anyway, after connecting them and listening, saying I was VERY disappointed would be an understatement. They sounded absolutely terrible - like my system was broken. I then set my streamer to play them around the clock for a couple of weeks, listening most days to see if there was any progress. At first, after a couple of days, they still sounded significantly worse than my other set of speaker cables, so I was tempted to just go back to them, but I persisted. After about 2 weeks of nearly constant playing, they finally opened up and had none of the grit I had heard. I'm now VERY happy - they sound great. After sitting idle for 20 years, breaking them in brought them back to life - YAY! 

@audphile1 

I usually completely eliminate the noise floor by turning off my amp

Ah, the ultimate proof that everything makes a difference 

@richardbrand indeed. 

here’s a crazy thing I’m trying to explain to myself…I have a theory or 3 but I won’t spill the beans yet…

here is a puzzle -

I have two power cords, Kimber PK10 Ascent in 2m length, PK14 Ascent in 1m. When I swap the 10 out with the 14 on my integrated, the sound becomes livelier, with faster transients and better instrument separation with the 14 gauge power cord. Same outlet and circuit. The sonic outcome is repeatable. I had no expectations for this swap it was done to speed up the power cord break in when I bought the 14. Consistent before, during and after break in.
Same results are obtained on my DAC but that’s even with the 2 cords of same length but different gauge (I have 2 PK14 cords one is 2m the other is 1m). Like I said I researched this after I thought about the reasons why to see if my thinking was in the right direction. 
I’ll add…I continue to use the 10 on the amp and 14 on DAC and phono stage. 

@audphile1 

Have you got any ferrite rings?  I am wondering what difference these might make placed around each end of a power cord.

I mention this because I have a KEF subwoofer in my motorhome.  The subwoofer with its built-in class D amplifier emitted so much EMI that it completely blocked digital TV reception, coming from the nearest tower.  Adding two ferrite rings to the subwoofer mains cable reduced the EMI and restored the TV signal.  This is completely objective and repeatable, no golden ears needed.  The effect operates regardless of whether the motorhome is connected to mains power or is running 'clean' power from batteries via a sine-wave inverter.

My thoughts are that mains cables can make a difference, but it has little to do with 'cleaning' up the incoming juice.  If there is significant EMI generated in a component, the power cable can act as an antenna.  Changing the length of an antenna changes its tuning!