this was also an issue for me when i was trying to get defcaped.. but i´t not that hard.. just trow a lil´ Get a blacksmith to do a helm for ya and put an Argent Dawn-Revered enchant on it.. just that will give you a major def boost.. look on my char on Armory (http://eu.wowarmory.com/character-sh...lade&n=Krothos) and i know you will get some good tips..
Sorry, i probably didn't express myself correctly, Defense is the only stat you can stack because of no Diminishing Returns on it.
I just meant that if you have 539 defense and the Raid leader won't invite you because of it, that would be strange.
If the raidleader has an alternative tank with 540 defense he would be extremely stupid to keep the crittable tank. no amount of stamina available on gear you would have prior to hitting 540 defense will save you if a boss decides to crit you (at least in a raid).