To answer the above questions - I'm nowhere near 50 hours yet.
I tried the sound from the amp direct through headphones today and was again disappointed. So I am starting to think about other causes. Apologies if what I am about to write is complete nonsense.
The room in which the Hifi is used has undergone a lot of changes recently, of necessity. A few weeds ago, we lost broadband and landline connections completely. It took two weeks to resolve the problem. It turned out that not only was there a fault outside the property, but inside it too. The wiring inside probably dated back to old GPO days, and there was tons of it around the property, supporting phones in various rooms. With the advent of cordless phones, all of that wiring became redundant but had been left in place. In addition, it was felt that the main phone socket in the house was poorly placed and it was changed, so that the telephone cable now enters the house in the room where the Hi-fi is situated.
This means that the base phone, which used to be in the hall, is now in the same room as the hi-fi too; and attached to that phone is one of those alarms that works with a pendant to alert someone when the wearer of the pendant suffers a fall. The broadband router is in this room too (but always was there, as indeed is my computer).
Since I have tried so many combinations with my hi-fi equipment, with the same disappointing results, I am now seriously wondering whether there is some interference generated from the other equipment in the room, which has an effect whatever combination of speakers, amps, headphones and CD players I use.
Is that complete nonsense, or is there likely to be some truth in it? Later on today, when I feel I can risk being unavailable, I shall remove this equipment and see if any positive changes result.