I asked Miothan for some help yesterday on locations of lootables by chest and discovered we likely don't have a completed list that is public at this point.
I began one yesterday with my finds while doing DF runs.
I am making it public and open to everyone to edit with correct information. It is a google spreadsheet.
Here's the link: https://docs.google.com/spreadsheets/d/1dW8PTu3ax8ebsJYnC5bmHmQrDdLtbiwF40k_y5N4D8A/edit?usp=sharing
Boss = Boss Chest
Challenge = Challenge Chest
Bosses drop either magnificent souls, angelic souls, or soul stones
Boss Chests can drop souls, soul stones, mythical skulls, or mythical items.
Challenge chests only drop mythical items
EDIT: 01/28/2015 - Clarifications
EDIT: 01/30/2015 - link is now "View Only" again. If you have some contributions to filling out the remainder of the missing items, please post them in this thread and I will add them. Also filled in the "Boss Drops" Tab and created a formatting scheme. Just finished formatting the look of the "Skull Drops" tab as well.
STILL TO DO:
1) Find remaining pieces of gear and their respective drop locations
2) Make the first post and the list standout more or potentially embed a view-only spreadsheet --> is this possible?
3) Odds and ends to improve everything