All potential bonuses

As far as I have read, different bonuses stack, while a double bonus (like 2x boss dmg, doesn't matter what % does not stack. About the result, on bosses you would either way see 126% damage (additive bonuses) or 127.2% (1.20 * 1.06). I don't know if bonuses add or multiply.

Double bonuses do stack. Things like %stat and %stat, or %int and %int stack additively, because they are the same bonus. The only double bonuses that don't stack are boss damage x2 and total damage x2.

To gain a multiplicative bonus, the bonus must be different (eg. %magic and %total damage). If total damage and boss damage stack, it's most likely going to be multiplicative, though it's possible that they might be additive since they both affect the "total" amount of damage dealt to a specific monster. However, it's still not confirmed whether or not they do stack anyways.