Here is a comparison of the error signal spectra after increasing the IMC modulation depth, to the contribution with RF inputs / whitening inputs terminated (which I borrowed from Koji's characterization of the same in Dec 2016, these shouldn't have changed).

Some general observations:

1. This data was taken with the WFS servos disabled, but with the IMC hand-aligned to a good state (MC_TRANS ~15,000). The error signal spectra are from the new DQ channels (but still sampled at 2048Hz, I had not implemented the change to 512Hz).
2. The error signals seem to have increased by ~25x , which is consistent with how much we expect the modulation depth to have increased
3. The bump around 1 Hz is now cleaerly visible in all 16 channels, as is the bounce peak at 16Hz (relative to Dec 2016). In general, between 0.1Hz and 5Hz, there is now a fair bit of daylight between the error signals and the electronics noise contribution. 

I will update with the in-loop error signal spectra, which should give us some idea of the loop bandwidth.

I will look into lowering the sampling rate, and how much out-of-band power is aliasing into the 0-256 Hz band and update with my findings.