1) Yes, all three tubes come into play. The third only as a high impedance load to the previous stage. EQ is dependent on the gain tubes, as rp is in parallel with the plate resistor and following loads (grid bias). In fact, my phono stages are far more dependent on tubes for EQ than probably any other. And that was on purpose.
2) Sure, you can use a low mu tube for output. Just ac couple it via capacitor.
3) Battery bias changes gain and rp just like a big capacitor would. It would greatly change EQ. It would also add distortion.
Don't know of any basic tube or phono books. And I'd rather not explain all of my secrets in one place. There is a lot of stuff going on in that little schematic, far more than is obvious. Tricks, etc. You can stare at it all day long and not see. Until I point it out. Then things become apparent. Each part is for a reason, everything playing together in harmony and balance.
Ok ok, I'll give you another example. The series resistor on the output. Seems rather meaningless, doesn't it? Very important. It is there to decouple the output cable (capacitance) from loading down the cathode follower too much at high frequencies. You see, a cathode follower has inherent degeneration and operates in a tight feedback loop. Not good to disrupt this feedback mechanism with extraneous loading, or it can lose stability. The sonics change. Adding that little resistance prevents a cable from affecting proper operation of the output stage. The price to pay is pretty small. Cathode followers have a bad reputation. This was one of the reasons.
jh
