A phono preamp adds equalization that a tuner doesn't need, so you won' hear normal frequency response if you do what you have proposed. You can do it, but don't crank the volume up.
As posted, this idea that a phono preamp needs to burn in is BS and if you contact the company, they'll tell you that it's absolutely necessary but the problem with that is:
Our memory for specifics about the sound isn't good enough to definitively say how or why it sounded different at 0 and 50 hours. People can think they can describe the differences, but they can't unless a component fails or the design has flaws.
What is the brand of the preamp and who modded it?