My solution to this whole problem would be as follows.
1) Make Adamantine armor negate crits and adamantine weapons auto crit
2) make the other helmets simply lower the damage taken on a crit. For example you could make the mundane METAL helms (white) do a -10% (rounded down) to all crits, uncommon Helms do -25% (rounded down) to all crits and in the case of Rare Helms (though the only one in the game atm IS the Grymskull Helm which is made of adamantine) gets -45% (rounded down) to all crits
