How to break in a tube preamp?

I own a linestage with teflon caps, but i would never say that they are universally good--it is all a matter of voicing. A cap that works well in one design, and to one person's taste can be really bad sounding in another application. I've heard gear "upgraded' with Blackgates and teflon caps that sounded really bad. I know of people that search for fifty year old caps that sound great in certain designs (another linestage I own uses these kinds of caps).

There really isn't such a thing as good caps and bad caps, it is more the case of finding the right cap.

To some extent, so-called "break in" is the listener becoming accustom to the new sound. That part requires listening to the system, so just do that and don't worry about break in.