If the preamp you are looking at does invert, in my opinion it is best to switch your speaker leads for optimum performance. Many people say that if the recording was not made in "absolute phase" that it does not matter. In all of my testing though, there is an audible difference if running an inverting pre without switching the leads. Also, I find that it is best to do this at the speaker itself.
What unit are you currently looking at if you don't mind me asking?