Okay, went back to the drawing board with Rana and Koji on PDH box stuff.

Currently (at least for the Yend), in the boost OFF state, we have an overall gain of about 50.  This is crazy big.  Also, the zero in the "transfer function stage" is around 1kHz, however our green cavity pole is (calculated) to be around 20 kHz.  Since these are supposed to cancel but they're not, we have a wide weird flat region in our loop TF.

So.  I calculated the changes to the TF stage that I'll need so that I have an increase of about 20 in DC gain, kept the pole at the same ~20Hz, but moved the zero way out to 18kHz.  I also calculated the changes needed for the integrator stage to make it effective at much higher frequency than it was designed for.  Now the pole is at 75 Hz, and the zero will be at 1.6kHz, and the high frequency gain will stay pretty close to the same with and without the boost.

Planned new TF stage:

Planned boost stage (with and without boost activated):

New boost stage only, so you can see the phase:

The schematic, modified to show my planned changes (which I will put in the DCC after I make the changes):