Come off your two feeds with 3.0 and into the new 3.0 mix valves, and be done with it.
You are in the drivers seat 100% here with the supply house, and they will know it when you start talking the technical aspects of this flawed design.
Just follow me here, and you'll get out of this all for the better. They won't screw with you, once they realize you know it is a flawed design on their part.
"He is far from Stupid"
”person, who happens to have an above-average level of intelligence”
