Gordon,
I did not expect that increasing the PIN diode bias would help.? I agree that there is a good chance that the imbalance causing the even harmonics could be in the driver stage.? I suggest measuring the voltage drop on the emitter resistors (R19, R20, R45, R48)?to see if one of the transistors is not pulling its fair share of the load.? You could also check the signals on the resistors with an oscilloscope to compare wave shapes.
73
Evan
AC9TU
