It may be outdated, but my few maxed Big 4 helped me get my first few EBs. *shrug*
To answer the original question: to unlock armor, you need to level it up as follows:
1/30 armor = level up to Lvl 15 (halfway)
1/50 armor = Lvl 25
1/70 armor = Lvl 35
That is to say, if I have an armor that maxes at Lvl 50, I need to raise it to Lvl 25 in order to unlock that particular armor in the future. NOTE: To actually make the +, you need to create a new one! after you've unlocked the first. The + will be the second one you make.
http://i57.tinypic.com/30074hs.jpg