TZ-20 Operating Specs?

I've gotten my TZ-20 RF amp up and running with my three tube 1934 transmitter but its not working like I expected.

I'm running with 500V on the push-pull plates with the recommended 2K bias resistor. Key down I'm seeing grid current (total for the two tubes) of about 40mA and total plate current of 40-50mA. Looking at the TZ-20 specs and tubes curves from the 1939 Taylor Tube manual I expected to see 50-75 watts input with 35-50 watts output, instead I'm seeing 20-25 watts input and only 8-10 watts output. I do have three TZ-20s. Any two of them behave the same. What an I doing wrong...or is it working as designed?