I've played a lot of epic tier 4e. PC ACs are in the same general vicinity as monsters. I think yiour calculation of the +6 Armour AC is not factoring in the level bonus. (Eg the 30h level paladin PC in my game wears plate armour and carries a shield and has an AC of 47; the scale-wearing fighter has an AC of 45.)In 4e a +6 Battleforged God Plate Armour gives you +20 to your AC (so base AC 30) which is much less then an Ancient Red Dragons AC of 48.