Thinking more after reading all the posts and what stands out:
1) Water changes really seem to help.
2) You actually do quite a few water changes as it is.
3) SPS are doing well.
4) You have tested all the basics, and they seem ok.
To me, that rules out all the normal pollutants, trace elements, and the simple things.
The basics would show on test kits, and the minor issues should have been taken care
of by all the water changes. And most simple pollutants would hit the SPS as well.
My opinion : It could be chemical warfare between corals, and the Montis are losing.
They are somewhat known to be weak in that area.
Have your corals grown together?
One thing to consider trying is aggressive carbon.