Rob and I went down to debug this asymmetric pedestal issue in MD2-. After seeing that the problem followed the Tube+PMT and not the preamp output we decided to try another spare base to see if we were unlucky and just have two bases that induce these 60 Hz transients.
Turns out that is indeed the case. Bases 121 and 122 seem to be flawed in the same way. Opening up the base that wasn't irradiated in Run 1 (121) didn't show any smoking barrels, but we'll take a closer look sometime in the near future.
Looking at the preamp outputs from both MD1+ and MD2- show reasonable looking symmetric pedestals, so it looks like this new base (123) is a keeper.
Will confirm once the parity DAQ is back in use.