Well I did the math, for the 1KW example 4100 vdc x .400 mA gives an impedance of 10,250 ohms, 250w would be about 2000 vdc x .200mA for an impedance of 10K. Dang! Thats neat.
That is also how the peak power for AM is 4 times the carrier. If you're running a tube at 2000v 200ma, when you modulate to 100%, the voltage will be double the supply voltage. Since the modulating impedance stays the same, the current also doubles, making 4 times the power that you would at carrier.