Driver/Input tubes

Download a copy of the schematic available at many websites.
The 12AU7 is an SRPP totem pole configuration voltage gain stage where the output is the midpoint between the first half triode's plate and the 2nd half triodes cathode.
The 12AT7 is 2 triodes wired in a differential amplifier config where the signal from the 12AU7 is split into two, faciliated by the 1Meg resistot that connects between the 2 grids. one phase that is used the "push" one of the KT88s and the other half triode is uised to "pull" the othet KT88.

Sorry I kept saying 12AU7 cos my personal unit is modded as a power amp not an integrated.

Let be go over this again.
Each channel has ONE 12AX7 as gain stage.
Each 12AX7 is setup in a totem pole configuration.
The output of the 12AX7 is connected to one triode of one of the triodes of the 12AT7s directly, then via a 1Meg resistor to the other triode of the 12AT7. The first one "pushes" the other one "pulls"
Each of the 12AT7s has its plate coupled using a coupling cap to a 6550, EL34, KT88 whatever you have in the amp.
Just download the schematic and spend 5 minutes looking at it and you wont think my explanation is too convoluted.
So why does it need 2 stages before the power tubes?
If the input were just the diffamp (12AT7) stage, then you will need a preamp, that swings 30-50V to drive that amp.
A CD player can only swing a few volts, so thats why the need for the 12AX7 gain stage.
The driver stage cannot have too much gain, otherwise it cannot deliver the current required to drive the power tubes and will "wimp out" and distort.