measure the voltage across the 2k resistor (actually with the trimmer resistance in parallel, it's closer to 1.67k), e/r gives you the current. now measure the total voltage from a cathode (does'nt matter which one if the tubes are balanced). e/i gives the total resistance, or since you have no bypass caps or any other reactive components here, it's also the impedance (i didn't take into account the gate capacitance of the fet, but if you use a fet with a small gate capacitance, it really shouldn't make much of a difference).