My level 8 Apkallu with: Defense Bonus (level 6), Damage Reduction (level 8) and Damage Reduction +15% was taking 10-20 damage off a friends level 9 Orobon. It's real hard to tell if they stack, but I managed to pull the fight back from me being at 60% HP and him being at like 95% HP due to Dread Spikes. He won with 1 hit's worth of HP left (1 single hit that is, not both of my apkallu's swings). Got another fight in 10 minutes, after that I might take off the Damage Reduction +15% because if they don't stack, I imagine it'll be the 2 damage reductions not stacking.
XI Wiki

