I've written down the drop rates from my last 17 runs (minus one Acheron where I had to leave unexpectedly). I'm about 10 years out of my last stats class but it seems like boots and chests might drop a little less than hands/belt and head/pants (4 feet from Dragon, 4 from Thanatos, 2 chests from KB, and 6 from Acheron). I've gone at least twenty runs now without a healer or tank chest dropping (which is like a .7% chance of happening if every item on KB has an equal chance of dropping). I don't know, I can post my loot so far on a google doc or something if anyone would find it helpful.