Have you ever hated the sound of your system one day, and loved it the next?

No, not ever. Not enthusiastic at times but never hate.

I’ve identified the problem. It’s a tube in the preamp that was going bad.
it was intermittently causing distortion and reeking havoc with the sound, from one day to another.  After replacing the tube, the set sounds better than ever (or so it seems after the nightmare.)

I believe attitude/frame of mind has a lot to do with it. If my mind is on other things, I do not get into as much as just  " chillin' and crankin' some tunes." However, like gillatgh, never hate it.

I had similar experiences regarding perceived SQ at different listening times...until I got serious about the setup and room treatment in my dedicated listening room. Made all the difference in the world.
The room will dictate half of what you hear.

This is normal. What you hear is subjective and will be heavily influenced by your mood and whether you have a cold, slept well or not etc.

Also much boutique equipment is finicky and very unreliable (like tubes).

This is probably the single biggest reason audiofools hear amazing things from fuses or ordinary bits of wire. The mood and expectation is extremely powerful in all humans. Foreplay is a good example. A finicky system that is on the fritz is yet another,

The trick is to recognize what is real from what is subjective and to restrict audio component purchases to reliable rather than boutique brands. Also avoid used gear that may have been damaged by misuse and non-qualified repairs. Those unable to do so are destined to keep buying new cables and performing countless new tweaks. The mind trick of the tweak creates the predisposition to feel more positive about the sound.