"I would really like to know what the 12AU7s do before the signal gets to the 12AX7s. And, which ones have the most effect on the sound output. "

I would imagine its only job is to manipulate the sound according to which tube is in the soket, and then output that sound to amp. I dont know that it does anything before the signal gets to it. But thats just my guess. I got my little knowledge for PoppaC, E-Stat, and Poultrygeist.

Did you see my post above of what PoppaChubby said?