Currently, on my assassin rogue (he's level 23, now, so much fun with twf agony), I'm using the shadowdancer epic destiny for its usefulness with assassins. However, there is a question about how much threat/aggro reduction there is and how much it stacks, if at all. I currently have:

Shadowdancer - rank 2 gives you Shadow Training II, which is a 20% threat reduction toggle.
Thief Acrobat - the tier 2 ability at rank 3, Subtlety, allows you to deal 40% less threat.
Nether Grasps - these gloves have Diversion on them, which reduces your threat by 20%.

Therein lies the problem: do these stack? Am I effectively getting 80% threat reduction on melee attacks or am I getting 40% because Subtlety is the highest value, or 60% because epic destiny passives usually stack with enhancements and/or items?