There was some uncertainty as to which channels were being input into the Adaptive Filtering screen, so I checked it out to confirm. As expected, the rows on the ASS_TOP_PEM screen directly correspond to the BNC inputs on the PEM_ADCU board in the 1Y6 (I think it's 6...) rack. So C1:ASS-TOP_PEM_1_INMON corresponds to the first BNC (#1) on the ADCU, etc.
After checking this out, I put text tags next to all the inputs on the ASS_TOP_PEM screen for all of the seismometers (which had not been there previously). Now it's nice and easy to select which witness channels you want to use for the adaptation.