Something you need to take into consideration here is that % mitigations happen before the damage formula.
So with something like Oni Hunter's at 9% the formula becomes:
(Damage x .91 x (100/(100+protections))
So when you're calculating how much it translates into, it doesn't actually increase with your protections, since you're going to mitigate the exact same amount regardless. For example, at 1000 damage, .91% is going to be 910 damage, meaning oni hunter's garb is always going to mitigate 90 damage, no matter your prots.
In your Bacchus example, % mitigation is additive. Bacchus' 10%, Spirit Robe's 15% and Oni hunter's 9% would add up to 34%, yes. In the same scenario of 1000 damage you're looking at:
(1000 x .66 x (100/(100+150)) = 264.
In reality, that's 73.6% reduction to the damage.
Edit: Whew, swapped out protections for damage there and added in a random 5% lol