As title states, what is the defense cap? If there is one or how much defense is too high before you start noticing any real difference from adding more defense.
This is conspiracy theory... Like damage one. For phy def, try add const 30(deff) in all slots of clothes and deco for a warrior and see how is it. Than look for relics and ads on. Add more.Put less and more and see how goes. You have more than one defense.