Found no power at the input to the two preamps serving aux003. Looks like that output of the preamp power distribution box (the output furthest to the left) has a blown fuse or suchlike. The adjacent output is okay. If another fuse goes I'll try to figure out why. (Recall that a different output on the distribution box has been powering two tee'd preamps for months.)
Went ahead and verified with voltmeter & current source that nothing strange happens in the DC limit when a small current is tee'd into these amplifiers. Now wait for beam.