The biggest problem I see is the step generator can only go to about -40V. Ten two Volt steps plus 20V of offset dialed in. I read about a guy in Yugoslavia that build an X10 amplifier to drive grids. It needs to be DC coupled with something like a -100V rail.
Pentode mode presents it's own problems. Most curves you see specify a DC Voltage for the screens. When the Plate Voltage collapses to zero each half cycle, the screen current can go to more than 100mA. You need a good adjustable current limit to avoid burning up the screens. You should seriously consider a series resistor on the screens. Occasionally you see UL mode curves, in that case the screen supply will be rectified AC of some percentage of the Plate supply.
As to the question about how close EL34's might match between triode and pentode mode, that's anybody's guess. You'll need a variac and a true RMS meter on the heater supply.
FYI, I own two 576's and a 570.