Phil that bridge rectifier is very similar to the one I used to fix one of of my 211 monoblocks a few months ago and documented on this forum.

I kinda feel your point that what's causing this issue is something that is failing when power is applied but looks fine when tested with power off.

You might not know I replaced all the output transistors and used what seemed to be incredibly expensive thermal paste. I went for the best stuff I could find, though.