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Some good points there. My thinking was that the loop was probably not
pretuned at all, or it would not have locked up properly with the incorrect
coil-sensitivity figure. But checking with the 8568 manual, it appears they
do pretune the main coil with a DAC to make sure it locks to the expected
20-MHz comb tooth.

That suggests that any sensitivity discrepancy is limited to the FM coil,
rather than the main coil. Not surprising, since main-coil sensitivity
seems to be 20 MHz/mA in many YTOs. Any temperature-related errors
introduced by shunt resistance across the FM coil should be corrected by the
PLL without difficulty.

At any rate, the objection I have to altering the loop filter is that it's
very hard to be sure you're not causing harm elsewhere. To make the
original PLL circuit work seamlessly with a different YIG, you need to
change the loop gain without altering its bandwidth, which can be tricky
considering that the gain and bandwidth normally changes over the tuning
range and, often, during the acquisition process itself.

Conversely, to fool the PLL into thinking it's controlling the old YTO, you
only have to change one parameter, the YTO's tuning sensitivity. If you're
lucky enough to have a more-sensitive YTO, adding shunt resistance across
the (non-pretuned) FM coil seems relatively safe...

-- john, KE5FX