Change seems effective. We just had ~2 hour lock during which we passed through the RH change transient peak and all PIs are dampable; we're no longer sitting over the 47kHz mechanical modes that were undampable this morning. We've been slowly changing modes over to PLL damping (as opposed to just static BP + phase filters). The potentially problematic Mode 2 (15520 Hz on ITMX) became unstable during the lock but remained damped with the PLL. We've confirmed that some too-high damping gain, even below ESD saturation values, will indeed make damping efforts ineffective; something along the way must be saturating. Damping using the PLL solves this as it outputs some fixed gain that we can keep below this 'bad' gain level (~30k cts).