Diablo – Which console versions of Diablo 3 ROS have Kanai’s Cube

There's a blue post over at the official forums trying to clear up some of the confusion. Essentially, those modes (Easy, Normal, Hard) do not alter Magic Find, Gold Find, and XP bonus in any way. They're just there for players to adjust the difficulty to their liking, increasing monster hit points and damage.

The Master I to Master V difficulties are different though:

• Master I: +60% Magic Find, +60% Gold Find, +120% XP Bonus
• Master II: +70% Magic Find, +70% Gold Find, +140% XP Bonus
• Master III: +80% Magic Find, +80% Gold Find, +160% XP Bonus
• Master IV: +90% Magic Find, +90% Gold Find, +180% XP Bonus
• Master V: +100% Magic Find, +100% Gold Find, +200% XP Bonus

That's pretty much the gist of it.

You need to hit level 10 to have access to the Master options.

Here's what I know from testing and reading the Diablo 3 forums: there are several groups of buffs and each group is multiplicative with the other groups (and thus a separate multiplier). Check out this thread for more information on multipliers.

To answer your specific question using the terminology from the linked thread:

1. The +10% fire damage from Ring A falls in Class B
2. The +15% fireball damage from Ring B falls in Class A3.

Assuming you're only wearing Ring A, then your 100 damage fireball with Fire element will hit for 100 * 1.1 = 110 damage.

Assuming you're only wearing Ring B, then your 100 damage fireball, regardless of element, will hit for 100 * 1.15 = 115 damage.

Assuming you're only wearing Ring A and Ring B and no other modifiying equipment, then your 100 damage fireball with Fire element will hit for 100 * 1.1 * 1.15 = 126.5 damage, which will be rounded to 127.

When you crit for 100% damage, it will do 100 * 1.1 * 1.15 * 2 = 253 damage.