I've implemented a more extensive feedback model that uses proper conversions between metres, volts, counts etc. and includes all the (inverse) (de)whitening filters, driver, servo and noise injections in the correct places. I then closed the loop to obtain the transfer function from horizontal ground motion and each noise source to test mass displacement. I tuned the servo gain to reduce the Q of both resonances to ~20.

Our idea was then to compensate servo gain with the output resistance of the coil driver to raise the RMS of the DAC output signal in order to raise SNR and thus suppress DAC noise coupling. I found that raising the output resistor by a factor of 10 above the nominal suggestion 2.4 kOhm gave me a DAC output RMS of 0.3 V, so in line with our safety factor of 10 requirements. This also coincidentally made all the noise sources intersect at approximately the same frequency when these noises begin to dominate over the seismic noise. All these initial tests are subject to change, particularly depending on the design of the servo transfer function. I'm attaching the relevant plots as well as the MATLAB script I used and the two files required for the script to run.

% Get piezo stack transfer function
PZT_f = fscanf(fopen('ground_freq.txt'), '%f');
PZT_tf = fscanf(fopen('ground_xx.txt'), '%f');

% Set frequency vector and ground motion
freq = logspace(-1, 2, 1e4);
grnd = ground(freq);
PZT = interp1(PZT_f, PZT_tf, freq);

% Set complex frequency variable

0.1
0.5
1
1.419178617
1.489659958
1.554545445
1.719720097
1.806748355
2.030363506
2.133112203

1
1.3
1.8
2.794167453
2.905480556
3.077890921
3.854210495
4.502922159
5.213856692
4.990356828