As you probably realised the formulae I posted are wrong - read (rp + Rp) for rp and they are OK.

On the subject of feedback, the possibilities are almost endless. As I was riding this morning I thought of an interesting possibility which is basically an extension of your E linear circuit. The driver transformer would have to have a split primary with each driver fed from an appropriate tap on the output transformer. The excess voltage would be dropped at the cathodes in the from of a large constant current source. 2A3s would probably not be the best way to go - say the 211s are running at 1000V then the CCS would have to absorb 120mA at 750 volts.

Probably better to run triode strapped pentodes say EL34s at 500V and 40 mA, meaning the CCS is now 500V at 80 mA, easily achieved with a cascode pentode / MOSFET CCS. The semi remote cutoff of the pentode will increase the third harmonic but this is inside your feedback loop so it shouldn't matter.