I presume you've tried different CDs and they're OK some times of the day and not at others.
What I would do here is to try and eliminate all common factors. So far you're tried different sources, amplifiers and 'speakers so we can assume it's not one of those. What about the cabling? Are those the same or have you changed interconnects and 'speaker cables? Unlikely as it may seem, it's possible I suppose for there to be an intermittent break/damage to the cabling that causes distortion rather than complete silence.
Once the entire system including cabling has been checked, the only thing remaining that I can think of is mains. I did think of external RF interference, but eliminated that as you've changed sources and amplifiers several times, and it's pretty unlikely for RF to affect three different amps (unless they are three examples of the same amplifier) .
Does this happen at specific times of the day, or is it random?
For mains, can you try running an extension cable from another ring in your house? If that's still a problem, is it possible to run a mains cable from a next door neighbour, as they may be on a separate phase of the mains. (Be careful with mains, especially over two phases.)
Finally, you may be in need of some mains filtration. I am very sceptical of any mains filter that's sold for HiFi, they are mostly overpriced and underperforming. Something like this might be more suitable.
schaffner - Search Results | CPC (farnell.com)
Schaffner are specialists in proper mains filtration, none of the audiophile bullshit that accompanies so much sold for HiFi.
Hope some of this helps. I can well understand how maddening this sort of thing can be.
S.