How do you break in a preamp

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.

You need I bought M8 preamp musical fidelity I was going to return it because don't like what I'm hearing but only 6 hours from what I'm reading is to soon to make a decision and return it hopefully with 50 more hours or more I be able to get the sound I expect for that kind of money

Agreed on leaving it on and using it. That's what I did for my XP-22 and I leave it on 24/7, I didn't notice a substantial difference as it broke in.

@stanwine congrats!!!

if you stream music just have your streamer playing 24/7. You don’t need to have the amp on. Also, keeping the preamp on even if not playing will help too. Just turn off the display when not listening (that’s what I do with my xp-12). 
You’ll need about 200hrs of break in.