Discussions with MIT (RichM) and looking closer at the very controllers & filters I plotted showed the problem.

While the developed controller I showed in the SEI Log, looked like it should not have been terrible at dealing with the potential instability at 8hz, the filter I posted in the LHO alog, was in fact a design unable to deal with this problem with strong peaking and marginal phase room at 8hz.  Once I realized these two views of the ETMY controller could not be the same, I knew somewhere the problem was twisted.  So I painstakingly went through all the filter design steps and sure enough, the design was there but was not saved correctly or prepared properly.  The Blue traces are the new X controller in the attached for ETMY while the Red traces are from the ITMX which is how ETMY appeared before.

There is of course no free lunch and if I understand and can explain (mostly for my own benefit).  The large phase bump in the 8ish Hz area pulls the controller away from instability and the design inherently has less gain peaking (maybe that is the same thing.)  On the other hand, if the zero in the plant (which can move around) moves away from this design point and/or the designer is too aggressive at matching the plant, the controller will develop something... (gain peaking?)

At the moment, ETMY is running under full Guardian Management in FULLY_ISOLATED.  Don't want to get ahead of myself but it has been almost 15 minutes now and Rai & Gerardo are still down there working on the chamber.  I have had one previous trip of the ISI for L4Cs but HEPI did not trip.  So, at least we are in a new epoch for BSC10 HEPI.