Low loss header question

I recently added a sunroom with a 2500 gal. spa and am having troubles with the boiler's header. It has three loops off of the primary. One to a plate exchanger heating the spa, one to a 10kbtu radiator, and one to 300' of 5/8 pex in the floor. They all work fine alone, but when all three are calling, the exchanger takes priority and drops the temp to the others. I can adjust the mixing valve on the pex loop, but then it is too hot when running alone. Would I be better off using a motorized mixing valve or should a factory header be used. I have been looking at a Caleffi Hydrolink Separator/ Manifold online and was wondering if anyone has experience with them.