I have used two purple tonics for the shadestalker helmet specifically, and while I dropped a few shadestalkers pieces of armor, I have not gotten a single helmet (shadestalker or otherwise). This seems like it should work different. I dunno if it's a question of luck or anything, but figured I'd give the info in case i'm not alone.
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I've tried a couple now. The offensive tonics seem to work, hard to confirm. The reward tonics definitely don't work or the drop rate is only increased by a small percent. Hoping for a bug or it will be a long four months unfortunately.