I just repaired the 4 bad preamps in my SQ5 by swapping the preamp printed circuit board. I have a theory about what may have caused the problem.
Here is a thread that sheds some light on this, though it may also add confusion without verification.
https://community.allen-heath.com/forums/topic/toggle-phantom-power
The 4 preamps I had a problem with were the only channels where I used phantom powered DI boxes. They were the Behringer Ultra DI DI100. I think the boxes sound great (transparent) but may have caused this problem. I have hot swapped phantom powered mics on other channels many times without a problem. I’ve swapped those DIs for passives after this thought.
It would be great if anyone else has info or experience to share.
Dean