Nope they dont stack.. They are permanent effects (voice of thule while the buff is up), but if you have a mask on that is Anger V (or 25% hate) then there is no need to cast Voice of thule buff..
its only needed to cast if you dont have a raid mask.. i think all the group masks are 22% hate, most raid are 25%, and i believe there is a TBM era anniv raid one that is 26% or 30% - the ONLY one i've ever seen..
Here is a decent thread, keep in mind its old and an emulator, but it also provides links to old steelwarrior and evilgamer (sadly not around anymore) that the information also came from for live. A lot of it is actual numbers from different hates. But VoT does stack with the worn mask. Why pallies and warriors always want VoT
We can't get hard numbers to compare, but you can create essentially identical situations to check how the aggro compares.
Zone in fresh to zone A, have a war with blast of anger a mob(not undead/animal) after pulling with proximity aggro. Use sk 1(mask only) to cast jolt spells until aggro flips with a set distance between the 3 entities.... Make note of how many casts.
Repeat after zoning out and back /camping with voice of Thule on also.
There is a big enough percentage that there should be a difference in casts. Alternatively you could use two sk and time chain castafter the war used a blast of anger and see who gets it first
Also, this would of been done in the past... Could also just set out to find that
The consensus from years ago on evilgamer/the steel warrior was that they stacked.
i stand corrected, but we could easily test it, 2 naked sks, with our lowest level terror spell, terrors a mob 10 times, then 2nd sk puts on mask and terrors the mob he should get aggro at under 10 casts, about 8th cast
now kill mob and repeat, but this time 2nd sk puts on mask and vot, he should get aggro in about 6th casts