Another Cary SLP 98 Tube question

I recently aquired a Cary SLP 98P, great pre. It uses 4 6sn7 tubes. I read (I believe in this forum) that the front tubes have one effect on the sound and the rears another. Okay what is the difference? I have a number of 6sn7's to roll through, but all pairs, no quads. So where do I begin front pair or rear?
Front pair which according to my inquiry with Cary is the gain stage. The rear tubes are the buffer.

Thank you for the response. Then rolling the front two makes more sense than the rears. What does the buffer do?