A trap one has to be careful not to fall into, is thinking that the quality of system playback is invariant, and that perceived differences must be due to one's hearing. I'm afraid all real system are constantly changing in quality, all the time - for myriads of reasons - though the changes may take place over longer periods of time. That the actual, measurable quality you hear at one time matches perfectly what it is on another occasion is a nonsense - I've spent years working on aspects of this " irritating behaviour" ...
An easy one that comes to mind: a friend I visit at times had sound which slowly deteriorated as we listened - over several minutes the sparkle just went out of the playback. He was totally unaware that this was happening, and I pointed it out, indicated what he should be listening for - and then he had no trouble picking it also. Long story short, he had a Naim integrated amp, with one of those "special" Alps blue volume pots - and the track was worn in the region where he normally set the gain. Adjusted system gain settings so that a different section of the pot track was used - problem went away! Which led to him ripping out that part, ordering a replacement, fitting it - and we were all happy again!
An easy one that comes to mind: a friend I visit at times had sound which slowly deteriorated as we listened - over several minutes the sparkle just went out of the playback. He was totally unaware that this was happening, and I pointed it out, indicated what he should be listening for - and then he had no trouble picking it also. Long story short, he had a Naim integrated amp, with one of those "special" Alps blue volume pots - and the track was worn in the region where he normally set the gain. Adjusted system gain settings so that a different section of the pot track was used - problem went away! Which led to him ripping out that part, ordering a replacement, fitting it - and we were all happy again!