• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

12AT7 vs 6SL7 as splitter for 6V6 ppp amp

Either of them have gobs of gain - that won't be an issue, but of the two, electrically the at7 better suits what you (sort of) describe you want. It has far more current capability than the sl7 and so will provide a firmer drive to the 6V6 grid.

Either a voltage amp to cathodyne or a ltp topology will work. If you choose the first, watch your heater to cathode voltages.