I made the plots a little nicer and added new sensor noises (from Brian Lantz's scripts and measurements). Click to enlarge.
The last plot shows that these other sensors' noises are lower than the differential ground motion below 1 Hz. Though 3 seismometers per STACIS is impractical, this shows that such seismometers could be used as feedforward sensors and provide isolation against differential ground motion. At these noise levels, the noise of the high voltage amplifier circuit in the STACIS would probably be the limiting factor.