How do threat modifiers stack?

[Warlock]

I recently ran across an unsupported statement on the Bliz forums that Blessing of Salvation does not stack with other sources of threat reduction.

While I've given that the credence that it deserves, it did get me thinking about how the various ways of reducing the threat you cause interact. Has anyone opened the hood on this and poked around?

For a warlock with full Nemesis, 5/5 Master Demologist (Imp) and Blessing of Salvation this could mean the difference between 70% threat reduction (linear stacking) and 20% threat reduction (use the first buff in the list). The most logical to my eye is that all would apply sequentially for a net of 55% threat reduction.

[Ruvanal]

Based on the way most things like this have been coded in the games and the way the data files label two of the effects, I would say it would be a straight linear add for a net -70% on the threat generated. Here are the listings for the 2 cases that do show the buried identiification for the mods:

http://www.thottbot.com/wow/?sp=23555
http://www.thottbot.com/wow/?sp=1038

Notice that these are referred to as additive modifiers. What can be important is that in many cases some of the modifiers are considered to be of the same class and that usually on one modifier of that class can be active at one time. Since I could not find a listing for which modifier the Master Demologist (Imp) using, it is possible that overlaps one of the other two but I doubt it. If it does, then at some point you might be getting a message that a more powerful effect is already active.

[lemekim]

As Ruvanal said, most of the percent-based modifiers stack linearly - attack speed, model size, etc. The only exception that I am aware of is % damage modifiers - they multiply instead of stacking linearly. It's hard to find a lot of examples since Blizzard likes to use +flat bonuses instead of percentages. And as Ruvanal mentioned, some spells are exclusive to each other as well.

But anyway, I would tend to agree here with Ruvanal that they stack linearly, as the majority does.

The only caveat here is that there might be a lower-end cap on the threat reduction, perhaps 50% ala weapon speed modifier. But then again, none of the other % modifiers do not have lower-end cap, so it's probably not there, but it is something you should be watching out for if you are planning to test it.

Oh, and it appears that imp thread reduction is there, just listed strangely on thottbot:

http://wowrunes.com/?wsid=23829
http://www.thottbot.com/?sp=23829