Is dirty power the culprit?

Remember that the human component (you) of your system is the biggest variable you have. Both your physical and mental condition affect what you perceive and those factors are constantly changing.

That is not to say your power supply couldn't be an issue, but if it is, you should be able to measure noise on the line with the appropriate equipment. A related factor could also be EMI/RFI noise that your system is picking up through the air. I've got a shortwave radio fellow who lives half a block from me and I can always tell when he's busy with his hobby, particularly if I'm playing LPs.

However, over the years, I've found I'm more often the weak link in the system than any component.

Are you playing the same source material when comparing?

Seems like an obvious thing but, if not, then this is a likely culprit. Not all recordings are created equal.

Mood/disposition towards listening is another likely factor.

Power? Maybe. That might be hard to prove though.

DOn't some power conditioning devices provide displays that indicate a measure of power "cleanliness" both in and out? If so, then maybe this could provide some real evidence that power is the culprit.

Also, any electronic device can have intermittent problems that prevent them from operating to spec at all times. Intermittent problems are the hardest to detect and reolve sometimes. You have to be able to swap pieces in and out one by one over a period of time and listen and compare to isolate a problem.

Simplicity can be your friend in resolving these kinds of issues. The more components in the signal chain, the more places there are where something can go wrong. Electronic gadgets are not perfect and do not always perform up to spec forever after all.

I have experienced the same problem and yes it is power related. Do a little research on EMI/RFI rejection and implementing a "Star Ground" system circuit. I no longer have the seesaw effect.

I would guess both power and the human factors cited above are likely culprits.

It could also be you just don't like your present system. I've had similar feelings about systems over the years, I'm finally at the point where I like my system all the time. While I may not enjoy it to the same level at each listening session, I never end my sessions feeling something is fundamentally wrong with the system, now chalk it up solely to mood/disposition variables.