How do you break in a preamp

Take the top plate off, most likley just some phillips head screws need to be removed.

What are you going to do once you are in there? :)

Fire it up and let play!! That's the only sure fire way to break-in any component. After 100-150 hrs. it should be sufficiently broke-in.

Yep, follow unit instructions: Power on, play. I personally don't like sitting through the break-in process because it's harder to perceive the differences over time. I prefer to listen at the beginning and take note of initial reactions, let it play for a decent period of time, then listen again for changes.

You can accomplish the burn-in process either way. I'm too impatient, and don't like to sit through the transformations that take place, to listen for 200 hrs while components burn. I just sample for a while every day. Whatever your signal source, your power amp needn't be turned on to burn-in a pre. To save the hours on my CDP(and it's tubes) I use a tuner, either between stations(white noise kinda), or on a "rap"(yuch!) station for the heavy bass content.

Well if we are talking about the new Pass Labs XP-10 (which I own) its designed to be left on all the time, there is no on /off button.
The manual says it reaches its full potential after 24 hours of being plugged in.
But if your like me , I would say give it a week of being on 24/7 before crtically judging it.

I broke mine in by having my cable TV output on a music channel running though it all the time.

Its really painless because it sounds pretty good right after hook up and does not go through massive changes with more hours on it.