When will a player begin to have a chance to receive equipment higher than Blue quality?

At present, I have have approximately 11,000 prestige, but the best weapons and rings I have found have been blue (Rare) quality, or the lesser green (Uncommon). Purple (Epic) amulets are an almost guaranteed drop from the last enemy in Group missions, but aside from them, I have not seen any epic level rings, weapons or additionals - nor have I seen yellow (Legendary) drops for any slot.

At what point may a player begin to have a chance at receiving the higher rarities? I figure that there's a prestige lock, as there is with the Silver Pommels, which unlock around 18,600 prestige.