Heaven or Hell

As others suggest, tt could be psychoacoustic, in other words, your state of mind changing from one listening session to another. Do you have dedicated lines? It could also be that you need to get dedicated lines to improve and make more stable the AC connected to your system. Personally, I've seen profound differences in the sound of my speakers based on any change to the AC powering the system.

Uprade your system big time then it will last longer .

It's hard to believe, but my system sounds the same every time I sit and listen.
I live out in the country with zero industies in the whole county. I'm the only house hooked to the transformer and I have two dedicated lines. The dedicated lines where an amazing difference. Even separating the digital from the anolog when I put the second dedicated line in was quit a large difference.
I'm not in the same mood everytime I listen, but the sound is the same.
I'm a firm believer in power coloration and addressing the issue. Power conditioners are a wonderful tool to help in this area.
I also don't want to sidestep the fact that the humor factor was way up there.
Needfreestuff was very funny in bringing us into the issue.
But Audiokinesis brought a instant laugh as soon as I read the first line. Well done.

It's not your system, its you. Don't feel ashamed, it happens to everyone (even though some don't want to admit it).

The difference in sound quality that you hear from day to day may not be psychological as another poster suggested . I used to have the same problem and felt the difference was to great to be a matter of state of mind ect . I attributed it to inconsistencies with incoming power . Upgrading to a top notch conditioner seemed to give me consistent sonic qualities . I haven't heard such anomalies since .