Today's seismometer diagnosis activities are still underway, this is just an update (since I did some reboots):
Problem 1: X and Z channels on both seismometers were flipped. I unplugged an X cable (East/West on the cable labels) and the Z channel (vert) would go to floating ADC zero. Rana suggested that the ADCs sometimes have random channel hopping, and that a reboot of the c0dcu1 computer which handles the PEM ADCU should fix this problem. I keyed the c0dcu1 / c0daqawg crate, those computers came back just fine, and the channels were no longer flipped. This is a good thing. Although now it's actually the Z channels that were / are bad on both seismometers, not the X's.
While rebooting those computers, c1iovme, c1sosvme, c1susvme1 and c1susvme2 crashed. I rebooted them, although for the first few power cycles c1susvme1&2 couldn't mount /cvs/cds/caltech. Eventually they did, and life is good again. Except that the seismometers are still funny.