If you notice such a change in performance then maybe you had one or more supplies blow a long time ago - if they blew one by one it finally got down to a point where it could not deliver enough power for the tube.

Since there seems to be no feedback or monitoring the system doesn't know there is a problem. Some supplies have a "output okay" connector or LED. Too bad GCC didn't incorporate some kind of circuit to do this. It doesn't seem like it would have cost a lot.