Double blind test- over a month- could this be a reliable test for any equipment?

And the sound changes all by itself day to day, hour to hour, month to month.

But if conducting the test over a month changes should average out. I would say if you cannot identify a difference in that scenario it is not worth the upgrade.

I think ideally it you could create a computer controlled AB that would only change (or not, double blind) after making a choice. Review the results and have an additional data point to make your decision.  You can always revert to feels if you like.

Summary of AES: Ten years of A/B/X Testing

https://www.audiosciencereview.com/forum/index.php?threads/aes-paper-digest-sensitivity-and-reliabil...