I've been designing and using audio equipment in my career for over 30 years and have never heard or measured any difference at all in the sound or performance of properly performing solid state (or IC op-amp based) electronics gear over time. Vacuum tube equipment does change it's sound over time for various reasons, but that mechanism is clearly understood.

I might ask, how rigorous a before/after comparison did you perform? Did you keep an "unused" example of your preamp for A/B comparison to the used one? If you did, was the comparison strictly double blind with the levels matched? Additionally, did you consider that your phono cartridge or the records themselves could have changed (worn) over time, and thus their sound would have changed? Both of these being mechanical in nature, such a change is possible.

It is far more likely that the "break-in" is simply your ears getting used to the sonic signature of your preamp as time passes.

If you are hearing a genuine change of the sound of a solid state component over time, that means that something in the circuit is changing, and that certainly is not good! A good design strives to remain stable over time, and unstable circuits are just not the norm.

I also might ask, how come every instance I hear about where somebody said their component had a "break-in", the change was always for the better? That doesn't make sense, wouldn't you think?