How about a performance based number of loot instead ?

You could add optionnal objectives to dungeons, that would simply increase the number of chests at the end, to encourage everyone to perform better.
- No death ? "Flawless victory bonus chest"
- Less than x number of damage taken ? "Life preservation bonus chest"
- x amount of total damage dealt ? "Brutal bonus chest"
- Every monster killed ? "Slayer bonus chest"
etc...