Hey Dingle! I'll take a stab at this - As far as the two different trodes as PIs go, there isn't a ton of difference between the two, really. The AT has a lower Plate Resistance, and can handle more Plate Power Dissipation.
Generally, if you want a clean amp, you taylor the gain stages through the amp to not overdrive any succeeding stages. You'd start at the tail end, and work back.
IOW, if your 6V6's (I'll assume Class A), has a Cathode Voltage of ~20V at idle, then you'd want the PI to make ~40V P-P at full volume. This will keep the output tubes from saturating. So, if the PI gain was ~20, then 2V input would be your limit there.
You could add a NFB loop, or you could convert it to Class AB with a fixed bias, or lower the gain(s) of the other stages.
I'm sure you'll get more suggestions on this.