Some brakes are more prone to squealing like a cat in heat... the old model of the Avid Shorty 4 was notorious as are the Oryx cantis while other models do not seem to experience the same problems and if they do, they can be rectified.

I had the Avid S4's on one of my bikes and they have been sitting on the shelf ever since I could not stand them any more and did switch them between several bikes and the squealing was constant... and I know how to set up cantis.

With that being said, my brother in law runs Oryx cantis on his LHT and I got a little low pressure squeal from them which is acceptable.

Seems like the OP has done everything except change out the cantis he has so this is probably the way to go.