Do you ever doubt you ears ?

Perception by its very nature defies certainty...that's what makes it interesting, and what makes music musical.

Do I trust my ears? I do and I don't. The problem to me, is what others have alluded to. I can always hear a difference in any change to my system. but is it better, or just different?

I believe our brains are hard wired to seek change, differnce, in what we see, hear etc. This means an inate prejudice to what is new. To me it nearly always sounds better. The only answer, as URu975 says, is does the change meet the test of time. Does a new component make you want to listen more over time? Are you listening to different music? In other words, has the change favoured a particular genre of music? For example, I have just added a SET amp to a hybrid integrated, both are wonderful, but I find myself picking out small scale Jazz or singer songwriter albuns, rather than classical symphonies.

>>Do you ever doubt your ears?<<

Never.

I don't doubt my ears when differences in the sound of components are to a very substantial degree. If I do catch the upgrade bug, it would be after several years of listening to my current system and then upgrading each component at a time, preferably at least a year apart. Before this, I read as many reviews as I can and then make a final decision based on my ears alone and how the new component synergises with the rest of the system.
To quote a character in the movie Ronin, "If there is any doubt, there is no doubt". So, don't make that upgrade if there is any doubt at all in your ears and in your mind about the "improvement" in sound. I find differences in digital components and speakers are more easy to detect by ear, probably because the technology would have improved by leaps and bounds after about 3 years with your old ones. Power amp differences are not so easy to hear.
But if there is an itch to upgrade, go for something that is several times the quality of the old one to remove any doubt(here is where all the reviews, especially the comparative ones, have helped me). If I can't afford the upgrade, I leave my current system alone for economic reasons.

A hifi item "Several times the quality" often means several times the price with a very small (or undetectable) improvement in sound...with something as difficult to quantify as "sound quality" I think we Audio Gear Hobbyists sometimes just change for change. I recently "upgraded" nearly everything in my stereo heap to relatively inexpensive used or discontinued different (but well regarded) things and it sounds a LOT better and for now I'm happy...but that pretty turntable at Goodwin's over there would make me a better person...and that thick, well reviewed power cable would make the electrons more organized and perkier...and most of my friends won't listen to any of it.