I have the uths connected to the rheostats shown in the 2nd pic in the 1st post. I setup the tub up as shown last night, and monitored it until I felt it was safe. I woke up this morning to hot side temps of 92, and cool side of 88. I turned the cool side down a bit, it's down to 86 and falling. So I think this setup works well.

As to the uths malfunctioning, does anyone actually know how hot they can get if they do malfunction?