The DARM error signal RMS is dominated by the large peak at 1.34 Hz. This is coherent with SRCL. I don't know what is the origin of the peak in SRCL, but today I did a quick test by turning off MICHFF and SRCLFF.

When SRCLFF is off, the peak is still present in SRCL, but it disappears from DARM. So the SRCLFF is injecting the peak in DARM.

One way to fix it (until we figure out why SRCL is moving that much at 1.34 Hz) would be to add a notch to the SRCLFF1 path. I have a design that should not change gain or phase of the SRCLFF abvoe 10 Hz in any significant way, while notching the 1.34 Hz peak by 20 db. And also a more aggressive high pass filter, that might work without spling the SRCL subtraction at 10Hz