I reinstalled the auxiliary 1811 on the input side of the table.
I tried to get the free-running noise of the north cavity by locking south and then using the PLL to read out the beat from the 1811. However, even on a FM deviation of 400 kHz / Vrms, I could not get the PLL to lock. So I suspect the free-running noise is just too high to use this method.
I also used this aux 1811 to optimize the PDH EOM alignments. I saw no change in the beat spectrum after doing this. I would like to demodulate the 1811 signal using the PDH LO, but this will require some reconfiguration of the RF distribution.