Gordon,
I think the first path to eliminate is the voltage doubler/HV link used to reverse bias the unused filters.? There is only the RFC to keep the signal path isolated.? If, when connecting that link, the second harmonic increases, then I would look to increase the RF suppression on that line.
If you still have the HV connected, then the solution is to use relays instead of the PIN diodes.
73
Evan
AC9TU
