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?
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