Hi Ross - I use a Dialogue 2. It came with KT88's, and I did change to some EL-34's, but I believe the Prologue 2 comes with EL-34's anyway. I would be interesting in hearing the other posters reasons for why the preamp tubes make a bigger difference than the power tubes? I have always heard the opposite. I have never tried switching the preamp tubes. The power tubes make a pretty big difference, at least between types.
Also, if this is a new unit, and you are using new tubes, there will be a "break-in" period of about 200 hours. I certainly wouldn't switch out any tubes before that point. You will definitely notice changes over that time in the tubes that are in there.