Its hard to believe that I'll have an original contribution to such a long-raging controversy, but here goes. A factor that the 'burn-in makes a difference' (bimad) supporters seem to overlook is that the same music from the same source presented by exactly the same equipment can be different each time you hear it. Let me offer 2 hypotheses to explain the difference that is heard:
1. This delta is due to subtle realignment of impurity atoms (or similar physical change) in a wire somewhere in your hifi system.
2. The listeners state of mind was different during the 2 listening experiences. This difference could be the result of expectation, attention level, increase or decrease in enthusiasm, etc. that subtly alters perception of very subjective sound details.
I favor the second explanation. A simple test you can try is to play the same piece of music 3 times in a row, and compare what is heard each time. I can almost guarantee that the 3rd cycle will not 'sound' identical to the first.

