Unfortunately, Dread Helms does not give a flat armor class bonus, as they are meant to be wearable no matter the set of armor that you have. Armor class magic items that stack on top of your normal armor are also extremely rare. Giving the helm that property would no longer make it a common magic item and would remove the reason for its existence: flavor.  While the Dread Helm doesn't have an armor class bonus, it doesn't matter anyway. You are getting one magical effect at no cost since it has no attunement cost. So count your blessings.