You're right, 90 psi is too much. A PRV is installed in the cold water supply prior to reaching the water heater. I really don't know about the filter situtation, someone else will have to answer that. You might contact the manufacturer or the dealer that handles that brand. You need to realize that a PRV creates a closed system so an expansion tank will be needed. We've had several discussions about them quite recently, you can scroll back and find them.