If you've got new electronic parts, the sound is very likely to change subtly over time.
I built my own amps from Nelson Pass designs and was able to compare two boards with identical designs, but with higher-quality parts in the new board. At first I was so disappointed with the new build because I spent so much money, time, and effort on the parts and it actually sounded worse than the original. Two weeks later it was leaps and bounds better, as I had originally hoped when I undertook the build. After that experience I'll never doubt the reality of burn-in again. Or parts quality for that matter.