Which module was it, I might have one not in use if you want to put it back into your original PO-400.
At this point, I WOULD NOT recommend adding more modules to your rack without some sort of power filtering circuitry on the breakout bus. It seems I’ve been lucky not having problems without it.
How many modules are in your rack that had the issue? Any specific power hungry/dirty modules you can think of? I’m thinking it fried at startup because the power fluctuated a lot. I think there are some specific tools to delay startup of modules to help with this.