I'm still having issues with the STACIS oscillating uncontrollably with the slightest extra vibration, but with some more added weight both x and z direction are stable if you don't disturb the setup.
I took more transfer functions of the STACIS. In the last data I took Jenne pointed out that the geophone signals were not correlated well with the driving signal, so I increased the amplitude of the driving signal and am looking in x and y too instead of just z.
Details of the driving signal: 25 mV, swept sine from 0.1 to 100 Hz from the SR785.
NOTE: The data below was all transferred from the SR785 using netGPIB, which works fine, if anyone was interested in using it.
Open loop in the y direction, taken with the y geophone (magnitude on top, phase on bottom):
Open loop in the x direction, taken with the x geophone (with some extra weight to try to make the closed loop more stable):
Open loop in the x direction, taken with accelerometer instead of geophone: