We've set up a beat note measurement between the VCO driver and the Marconi (see Suresh's elog).
Here's the 'unWhiten' filter for compensating the SR560 TF.
It has poles = 1 mHz, 5 kHz, 5 kHz
and zeros = 30 mHz, 1 kHz
The gain is set to be ~0.001 in the 1-100 Hz band to compensate the G=1000 of the SR560.