The massive bonus to AR/DR for your current target is 50/50, leading to a 100 point advantage. The passive bonus is a bonus 10 AR per tier you have unlocked. The total bonuses show up in the Equipment comparison for your SC in a fight. So far it is confirmed that tiers are unlocked at 10/50/100/500/1000/5000 kills. Your passive and active bonuses do stack. It is notable that you only unlock your next tier by heading back to the tavern.
An interesting aspect of Legendary Hunter is that you get an experience reward when unlocking each tier for each creature type. The amount of bonus XP given depends on both the tier and your current level when that tier is unlocked, and the formula is the following:
-TIER 1 (10 kills): 100 x level
-TIER 2 (50 kills): 200 x level
-TIER 3 (100 kills): 1000 x level
-TIER 4 (500 kills): 2000 x level
-TIER 5 (1000 kills): 10000 x level
-TEIR 6 (5000 kills): (Given previous pattern, it is estimated that this will be 20000 x level, please verify when known)
Now all of that said, because you tend to face more variety of creatures outside of sub-locations (sub-locations tend to include a specific creature type predominantly ), it is highly recommended to get Legendary Explorer before Legendary Hunter. It is also recommended that you get this power early; it takes a lot of fights through several remorts to get all of the creature types up to a high tier, so the sooner you can start on that the better.